gdb: don't use -Wmissing-prototypes with g++

This commit aims to not make use of -Wmissing-prototypes when
compiling with g++.

Use of -Wmissing-prototypes was added with this commit:

  commit a0761e34f054767de6d6389929d27e9015fb299b
  Date:   Wed Mar 11 15:15:12 2020 -0400

      gdb: enable -Wmissing-prototypes warning

Because clang can provide helpful warnings with this flag.
Unfortunately, g++ doesn't accept this flag, and will give this
warning:

  cc1plus: warning: command line option ‘-Wmissing-prototypes’ is valid for C/ObjC but not for C++

In theory the fact that this flag is not supported should be detected
by the configure check in gdbsupport/warning.m4, but for users of
ccache, this check doesn't work due to a long standing ccache issue:

  https://github.com/ccache/ccache/issues/738

The ccache problem is that -W... options are reordered on the command
line, and so -Wmissing-prototypes is seen before -Werror.  Usually
this doesn't matter, but the above warning (about the flag not being
valid) is issued before the -Werror flag is processed, and so is not
fatal.

There have been two previous attempts to fix this that I'm aware of.
The first is:

  https://sourceware.org/pipermail/gdb-patches/2021-September/182148.html

In this attempt, instead of just relying on a compile to check if a
flag is valid, the proposal was to both compile and link.  As linking
doesn't go through ccache, we don't suffer from the argument
reordering problem, and the link phase will correctly fail when using
-Wmissing-prototypes with g++.  The configure script will then disable
the use of this flag.

This approach was rejected, and the suggestion was to only add the
-Wmissing-prototypes flag if we are compiling with gcc.

The second attempt, attempts this approach, and can be found here:

  https://sourceware.org/pipermail/gdb-patches/2021-November/183076.html

This attempt only adds the -Wmissing-prototypes flag is the value of
GCC is not 'yes'.  This feels like it is doing the right thing,
unfortunately, the GCC flag is really a 'is gcc like' flag, not a
strict, is gcc check.  As such, GCC is set to 'yes' for clang, which
would mean the flag was not included for clang or gcc.  The entire
point of the original commit was to add this flag for clang, so
clearly the second attempt is not sufficient either.

In this new attempt I have added gdbsupport/compiler-type.m4, this
file defines AM_GDB_COMPILER_TYPE.  This macro sets the variable
GDB_COMPILER_TYPE to either 'gcc', 'clang', or 'unknown'.  In future
the list of values might be extended to cover other compilers, if this
is ever useful.

I've then modified gdbsupport/warning.m4 to only add the problematic
-Wmissing-prototypes flag if GDB_COMPILER_TYPE is not 'gcc'.

I've tested this with both gcc and clang and see the expected results,
gcc no longer attempts to use the -Wmissing-prototypes flag, while
clang continues to use it.

When compiling using ccache, I am no longer seeing the warning.

12 files changed, 275 insertions, 6 deletions

diff --git a/gdb/acinclude.m4 b/gdb/acinclude.m4
index 2bdc1ce..95ff2b6 100644
--- a/gdb/acinclude.m4
+++ b/gdb/acinclude.m4
@@ -15,6 +15,9 @@ m4_include(acx_configure_dir.m4)
 # This gets GDB_AC_TRANSFORM.
 m4_include(transform.m4)
 
+# This get AM_GDB_COMPILER_TYPE.
+m4_include(../gdbsupport/compiler-type.m4)
+
 # This gets AM_GDB_WARNINGS.
 m4_include(../gdbsupport/warning.m4)
 
diff --git a/gdb/configure b/gdb/configure
index 429eaeb..e751edc 100755
--- a/gdb/configure
+++ b/gdb/configure
@@ -17000,6 +17000,64 @@ _ACEOF
 
 
 
+
+  { $as_echo "$as_me:${as_lineno-$LINENO}: checking the compiler type" >&5
+$as_echo_n "checking the compiler type... " >&6; }
+if ${gdb_cv_compiler_type+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  gdb_cv_compiler_type=unknown
+  if test "$gdb_cv_compiler_type" = unknown; then
+     cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+
+                          #if !defined __GNUC__ || defined __clang__
+                          #error not gcc
+                          #endif
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  gdb_cv_compiler_type=gcc
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+  fi
+
+  if test "$gdb_cv_compiler_type" = unknown; then
+     cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+
+                          #ifndef __clang__
+                          #error not clang
+                          #endif
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  gdb_cv_compiler_type=clang
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+  fi
+
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $gdb_cv_compiler_type" >&5
+$as_echo "$gdb_cv_compiler_type" >&6; }
+
+ GDB_COMPILER_TYPE="$gdb_cv_compiler_type"
+
+
 # Check whether --enable-werror was given.
 if test "${enable_werror+set}" = set; then :
   enableval=$enable_werror; case "${enableval}" in
@@ -17036,10 +17094,16 @@ build_warnings="-Wall -Wpointer-arith \
 -Wdeprecated-copy-dtor \
 -Wredundant-move \
 -Wmissing-declarations \
--Wmissing-prototypes \
 -Wstrict-null-sentinel \
 "
 
+# The -Wmissing-prototypes flag will be accepted by GCC, but results
+# in a warning being printed about the flag not being valid for C++,
+# this is something to do with using ccache, and argument ordering.
+if test "$GDB_COMPILER_TYPE" != gcc; then
+  build_warnings="$build_warnings -Wmissing-prototypes"
+fi
+
 case "${host}" in
   *-*-mingw32*)
     # Enable -Wno-format by default when using gcc on mingw since many
diff --git a/gdb/configure.ac b/gdb/configure.ac
index 13e880d..5a380ce 100644
--- a/gdb/configure.ac
+++ b/gdb/configure.ac
@@ -1833,6 +1833,7 @@ GDB_AC_WITH_DIR(SYSTEM_GDBINIT_DIR, system-gdbinit-dir,
     [automatically load system-wide gdbinit files from this directory],
     [])
 
+AM_GDB_COMPILER_TYPE
 AM_GDB_WARNINGS
 AM_GDB_UBSAN
 
diff --git a/gdbserver/acinclude.m4 b/gdbserver/acinclude.m4
index d68e318..32d5d14 100644
--- a/gdbserver/acinclude.m4
+++ b/gdbserver/acinclude.m4
@@ -7,6 +7,9 @@ dnl system search paths.
 dnl gdb/gdbserver/configure.in uses BFD_HAVE_SYS_PROCFS_TYPE.
 m4_include(../bfd/bfd.m4)
 
+# This get AM_GDB_COMPILER_TYPE.
+m4_include(../gdbsupport/compiler-type.m4)
+
 dnl This gets AM_GDB_WARNINGS.
 m4_include(../gdbsupport/warning.m4)
 
diff --git a/gdbserver/configure b/gdbserver/configure
index d90135b..bcb1594 100755
--- a/gdbserver/configure
+++ b/gdbserver/configure
@@ -9707,6 +9707,64 @@ fi
 
 
 
+
+  { $as_echo "$as_me:${as_lineno-$LINENO}: checking the compiler type" >&5
+$as_echo_n "checking the compiler type... " >&6; }
+if ${gdb_cv_compiler_type+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  gdb_cv_compiler_type=unknown
+  if test "$gdb_cv_compiler_type" = unknown; then
+     cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+
+                          #if !defined __GNUC__ || defined __clang__
+                          #error not gcc
+                          #endif
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  gdb_cv_compiler_type=gcc
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+  fi
+
+  if test "$gdb_cv_compiler_type" = unknown; then
+     cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+
+                          #ifndef __clang__
+                          #error not clang
+                          #endif
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  gdb_cv_compiler_type=clang
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+  fi
+
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $gdb_cv_compiler_type" >&5
+$as_echo "$gdb_cv_compiler_type" >&6; }
+
+ GDB_COMPILER_TYPE="$gdb_cv_compiler_type"
+
+
 # Check whether --enable-werror was given.
 if test "${enable_werror+set}" = set; then :
   enableval=$enable_werror; case "${enableval}" in
@@ -9743,10 +9801,16 @@ build_warnings="-Wall -Wpointer-arith \
 -Wdeprecated-copy-dtor \
 -Wredundant-move \
 -Wmissing-declarations \
--Wmissing-prototypes \
 -Wstrict-null-sentinel \
 "
 
+# The -Wmissing-prototypes flag will be accepted by GCC, but results
+# in a warning being printed about the flag not being valid for C++,
+# this is something to do with using ccache, and argument ordering.
+if test "$GDB_COMPILER_TYPE" != gcc; then
+  build_warnings="$build_warnings -Wmissing-prototypes"
+fi
+
 case "${host}" in
   *-*-mingw32*)
     # Enable -Wno-format by default when using gcc on mingw since many
diff --git a/gdbserver/configure.ac b/gdbserver/configure.ac
index 5576546..dc33f1a 100644
--- a/gdbserver/configure.ac
+++ b/gdbserver/configure.ac
@@ -146,6 +146,7 @@ fi
 AC_SUBST(ustlibs)
 AC_SUBST(ustinc)
 
+AM_GDB_COMPILER_TYPE
 AM_GDB_WARNINGS
 
 dnl dladdr is glibc-specific.  It is used by thread-db.c but only for
diff --git a/gdbsupport/Makefile.in b/gdbsupport/Makefile.in
index 36c2e3d..c74e26a 100644
--- a/gdbsupport/Makefile.in
+++ b/gdbsupport/Makefile.in
@@ -123,8 +123,8 @@ am__aclocal_m4_deps = $(top_srcdir)/../config/codeset.m4 \
 	$(top_srcdir)/../config/ax_pthread.m4 \
 	$(top_srcdir)/../gdb/ax_cxx_compile_stdcxx.m4 \
 	$(top_srcdir)/../gdb/libiberty.m4 $(top_srcdir)/selftest.m4 \
-	$(top_srcdir)/ptrace.m4 $(top_srcdir)/warning.m4 \
-	$(top_srcdir)/configure.ac
+	$(top_srcdir)/ptrace.m4 $(top_srcdir)/compiler-type.m4 \
+	$(top_srcdir)/warning.m4 $(top_srcdir)/configure.ac
 am__configure_deps = $(am__aclocal_m4_deps) $(CONFIGURE_DEPENDENCIES) \
 	$(ACLOCAL_M4)
 DIST_COMMON = $(srcdir)/Makefile.am $(top_srcdir)/configure \
diff --git a/gdbsupport/acinclude.m4 b/gdbsupport/acinclude.m4
index cc9c2dc..c627614 100644
--- a/gdbsupport/acinclude.m4
+++ b/gdbsupport/acinclude.m4
@@ -6,5 +6,8 @@ m4_include([../gdb/libiberty.m4])
 m4_include([selftest.m4])
 m4_include([ptrace.m4])
 
+dnl This gets AM_GDB_COMPILER_TYPE.
+m4_include(compiler-type.m4)
+
 dnl This gets AM_GDB_WARNINGS.
 m4_include(warning.m4)
diff --git a/gdbsupport/compiler-type.m4 b/gdbsupport/compiler-type.m4
new file mode 100644
index 0000000..f943a98
--- /dev/null
+++ b/gdbsupport/compiler-type.m4
@@ -0,0 +1,59 @@
+dnl Autoconf configure script for GDB, the GNU debugger.
+dnl Copyright (C) 2022 Free Software Foundation, Inc.
+dnl
+dnl This file is part of GDB.
+dnl
+dnl This program is free software; you can redistribute it and/or modify
+dnl it under the terms of the GNU General Public License as published by
+dnl the Free Software Foundation; either version 3 of the License, or
+dnl (at your option) any later version.
+dnl
+dnl This program is distributed in the hope that it will be useful,
+dnl but WITHOUT ANY WARRANTY; without even the implied warranty of
+dnl MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+dnl GNU General Public License for more details.
+dnl
+dnl You should have received a copy of the GNU General Public License
+dnl along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Sets up GDB_COMPILER_TYPE to either 'gcc', 'clang', or 'unknown'.
+# The autoconf compiler check will set GCC=yes for clang as well as
+# gcc, it's really more of a "is gcc like" check.
+#
+# By contrast, this will set the GDB_COMPILER_TYPE to 'gcc' only for
+# versions of gcc.
+#
+# There's no reason why this can't be extended to identify other
+# compiler types if needed in the future, users of this variable
+# should therefore avoid relying on the 'unknown' value, instead
+# checks should be written in terms of the known compiler types.
+AC_DEFUN([AM_GDB_COMPILER_TYPE],[
+
+  AC_CACHE_CHECK([the compiler type],
+                 [gdb_cv_compiler_type],
+ [gdb_cv_compiler_type=unknown
+  if test "$gdb_cv_compiler_type" = unknown; then
+     AC_COMPILE_IFELSE(
+        [AC_LANG_PROGRAM([],
+                         [
+                          #if !defined __GNUC__ || defined __clang__
+                          #error not gcc
+                          #endif
+                         ])],
+        [gdb_cv_compiler_type=gcc], [])
+  fi
+
+  if test "$gdb_cv_compiler_type" = unknown; then
+     AC_COMPILE_IFELSE(
+        [AC_LANG_PROGRAM([],
+                         [
+                          #ifndef __clang__
+                          #error not clang
+                          #endif
+                         ])],
+        [gdb_cv_compiler_type=clang], [])
+  fi
+ ])
+
+ GDB_COMPILER_TYPE="$gdb_cv_compiler_type"
+])
diff --git a/gdbsupport/configure b/gdbsupport/configure
index 8b4cb7e..afaba88 100755
--- a/gdbsupport/configure
+++ b/gdbsupport/configure
@@ -10192,6 +10192,64 @@ fi
 
 # Detect support warning flags.
 
+
+  { $as_echo "$as_me:${as_lineno-$LINENO}: checking the compiler type" >&5
+$as_echo_n "checking the compiler type... " >&6; }
+if ${gdb_cv_compiler_type+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  gdb_cv_compiler_type=unknown
+  if test "$gdb_cv_compiler_type" = unknown; then
+     cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+
+                          #if !defined __GNUC__ || defined __clang__
+                          #error not gcc
+                          #endif
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  gdb_cv_compiler_type=gcc
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+  fi
+
+  if test "$gdb_cv_compiler_type" = unknown; then
+     cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+
+                          #ifndef __clang__
+                          #error not clang
+                          #endif
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  gdb_cv_compiler_type=clang
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+  fi
+
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $gdb_cv_compiler_type" >&5
+$as_echo "$gdb_cv_compiler_type" >&6; }
+
+ GDB_COMPILER_TYPE="$gdb_cv_compiler_type"
+
+
 # Check whether --enable-werror was given.
 if test "${enable_werror+set}" = set; then :
   enableval=$enable_werror; case "${enableval}" in
@@ -10228,10 +10286,16 @@ build_warnings="-Wall -Wpointer-arith \
 -Wdeprecated-copy-dtor \
 -Wredundant-move \
 -Wmissing-declarations \
--Wmissing-prototypes \
 -Wstrict-null-sentinel \
 "
 
+# The -Wmissing-prototypes flag will be accepted by GCC, but results
+# in a warning being printed about the flag not being valid for C++,
+# this is something to do with using ccache, and argument ordering.
+if test "$GDB_COMPILER_TYPE" != gcc; then
+  build_warnings="$build_warnings -Wmissing-prototypes"
+fi
+
 case "${host}" in
   *-*-mingw32*)
     # Enable -Wno-format by default when using gcc on mingw since many
diff --git a/gdbsupport/configure.ac b/gdbsupport/configure.ac
index 4222bb7..55477d5 100644
--- a/gdbsupport/configure.ac
+++ b/gdbsupport/configure.ac
@@ -57,6 +57,7 @@ AM_CONDITIONAL(SELFTEST, $enable_unittests)
 GDB_AC_PTRACE
 
 # Detect support warning flags.
+AM_GDB_COMPILER_TYPE
 AM_GDB_WARNINGS
 
 case ${host} in
diff --git a/gdbsupport/warning.m4 b/gdbsupport/warning.m4
index a6b06e8..e9025db 100644
--- a/gdbsupport/warning.m4
+++ b/gdbsupport/warning.m4
@@ -51,10 +51,16 @@ build_warnings="-Wall -Wpointer-arith \
 -Wdeprecated-copy-dtor \
 -Wredundant-move \
 -Wmissing-declarations \
--Wmissing-prototypes \
 -Wstrict-null-sentinel \
 "
 
+# The -Wmissing-prototypes flag will be accepted by GCC, but results
+# in a warning being printed about the flag not being valid for C++,
+# this is something to do with using ccache, and argument ordering.
+if test "$GDB_COMPILER_TYPE" != gcc; then
+  build_warnings="$build_warnings -Wmissing-prototypes"
+fi
+
 case "${host}" in
   *-*-mingw32*)
     # Enable -Wno-format by default when using gcc on mingw since many