Fallback to stub-termcap.c on all hosts

Currently building gdb is impossible without an installed termcap or
curses library.  But, GDB already has a very minimal termcap in the
tree to handle this situation for Windows -- gdb/stub-termcap.c.  This
patch makes that the fallback for all hosts.

Testing this on GNU/Linux (by simply hacking away the termcap/curses
detection in gdb/configure.ac), we trip on:

 ../readline/libreadline.a(terminal.o): In function `_rl_init_terminal_io':
 /home/pedro/gdb/mygit/src/readline/terminal.c:527: undefined reference to `PC'
 /home/pedro/gdb/mygit/src/readline/terminal.c:528: undefined reference to `BC'
 /home/pedro/gdb/mygit/src/readline/terminal.c:529: undefined reference to `UP'
 /home/pedro/gdb/mygit/src/readline/terminal.c:538: undefined reference to `PC'
 /home/pedro/gdb/mygit/src/readline/terminal.c:539: undefined reference to `BC'
 /home/pedro/gdb/mygit/src/readline/terminal.c:540: undefined reference to `UP'

These are globals that are normally defined by termcap (or ncurses'
termcap emulation).

Now, we could just define replacements in stub-termcap.c, but
readline/terminal.c (at least the copy in our tree) has this:

 #if !defined (__linux__) && !defined (NCURSES_VERSION)
 #  if defined (__EMX__) || defined (NEED_EXTERN_PC)
 extern
 #  endif /* __EMX__ || NEED_EXTERN_PC */
 char PC, *BC, *UP;
 #endif /* !__linux__ && !NCURSES_VERSION */

which can result in readline defining the globals too.  That will
usually work out in C, given that "-fcommon" is usually the default
for C compilers, but that won't work for C++, or C with -fno-common
(link fails with "multiple definition" errors)...

Mirroring those #ifdef conditions in the stub termcap screams
"brittle" to me -- I can see them changing in latter readline
versions.

Work around that by simply using __attribute__((weak)).
Windows/PE/COFF's do support weak, but not on gcc 3.4 based toolchains
(4.8.x does work).  Given the file never needed the variables while it
was Windows-only, just continue not defining them there.  All other
supported hosts should support this.

gdb/ChangeLog:
2015-04-06  Pedro Alves  <palves@redhat.com>
	    Bernd Edlinger  <bernd.edlinger@hotmail.de>

	* configure.ac: Remove the mingw32-specific stub-termcap.o
	fallback, and instead fallback to the stub termcap on all hosts.
	* configure: Regenerate.
	* stub-termcap.c [!__MINGW32__] (PC, BC, UP): Define as weak
	symbols.

4 files changed, 31 insertions, 13 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index fed4e9e..919e93c 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,12 @@
+2015-04-06  Pedro Alves  <palves@redhat.com>
+	    Bernd Edlinger  <bernd.edlinger@hotmail.de>
+
+	* configure.ac: Remove the mingw32-specific stub-termcap.o
+	fallback, and instead fallback to the stub termcap on all hosts.
+	* configure: Regenerate.
+	* stub-termcap.c [!__MINGW32__] (PC, BC, UP): Define as weak
+	symbols.
+
 2015-04-03  Pierre-Marie de Rodat  <derodat@adacore.com>
 
 	* gdbtypes.c (is_dynamic_type_internal): Remove the unused
diff --git a/gdb/configure b/gdb/configure
index b8d4a7a..f835157 100755
--- a/gdb/configure
+++ b/gdb/configure
@@ -7455,11 +7455,6 @@ case $host_os in
   go32* | *djgpp*)
     ac_cv_search_tgetent="none required"
     ;;
-  *mingw32*)
-    if test x"$curses_found" != xyes; then
-      ac_cv_search_tgetent="none required"
-      CONFIG_OBS="$CONFIG_OBS stub-termcap.o"
-    fi ;;
 esac
 
 # These are the libraries checked by Readline.
@@ -7521,7 +7516,7 @@ fi
 
 
 if test "$ac_cv_search_tgetent" = no; then
-  as_fn_error "no termcap library found" "$LINENO" 5
+  CONFIG_OBS="$CONFIG_OBS stub-termcap.o"
 fi
 
 
diff --git a/gdb/configure.ac b/gdb/configure.ac
index ace033e..c703e35 100644
--- a/gdb/configure.ac
+++ b/gdb/configure.ac
@@ -617,18 +617,13 @@ case $host_os in
   go32* | *djgpp*)
     ac_cv_search_tgetent="none required"
     ;;
-  *mingw32*)
-    if test x"$curses_found" != xyes; then
-      ac_cv_search_tgetent="none required"
-      CONFIG_OBS="$CONFIG_OBS stub-termcap.o"
-    fi ;;
 esac
 
 # These are the libraries checked by Readline.
 AC_SEARCH_LIBS(tgetent, [termcap tinfo curses ncurses])
 
 if test "$ac_cv_search_tgetent" = no; then
-  AC_MSG_ERROR([no termcap library found])
+  CONFIG_OBS="$CONFIG_OBS stub-termcap.o"
 fi
 
 AC_ARG_WITH([system-readline],
diff --git a/gdb/stub-termcap.c b/gdb/stub-termcap.c
index 5897d89..722929f 100644
--- a/gdb/stub-termcap.c
+++ b/gdb/stub-termcap.c
@@ -40,9 +40,28 @@ extern char *tgoto (const char *cap, int col, int row);
 }
 #endif
 
+/* These globals below are global termcap variables that readline
+   references.
+
+   Actually, depending on preprocessor conditions that we don't want
+   to mirror here (as they may change depending on readline versions),
+   readline may define these globals as well, relying on the linker
+   merging them if needed (-fcommon).  That doesn't work with
+   -fno-common or C++, so instead we define the symbols as weak.
+   Don't do this on Windows though, as MinGW gcc 3.4.2 doesn't support
+   weak (later versions, e.g., 4.8, do support it).  Given this stub
+   file originally was Windows only, and we only needed this when we
+   made it work on other hosts, it should be OK.  */
+#ifndef __MINGW32__
+char PC __attribute__((weak));
+char *BC __attribute__((weak));
+char *UP __attribute__((weak));
+#endif
+
 /* Each of the files below is a minimal implementation of the standard
    termcap function with the same name, suitable for use in a Windows
-   console window.  */
+   console window, or when a real termcap/curses library isn't
+   available.  */
 
 int
 tgetent (char *buffer, char *termtype)