Allow overriding CFLAGS and similar when building

Before this patch, I don't think it was possible to change (say)
CFLAGS as part of running the make command. Nor did setting them when
running configure do anything. Getting this right is a little fiddly:
for example, see Automake's approach at [1] ("AM_CFLAGS" and friends).

This patch adds an "mcppbs-" prefix, and sets things up properly for
CFLAGS, CPPFLAGS, CXXFLAGS and LDFLAGS. Note that the bulk of the
patch is either the auto-generated configure script or the ax_*.m4
files vendored in from the autoconf archive (needed to handle
--export-dynamic correctly without trashing settings from the user
running configure).

What's supposed to happen is as follows:

  - Base compilation flags that should apply to everything (standard
    optimisation flags, warning flags etc.) are defined in
	Makefile.in.

  - When the user runs configure, they can set compilation flags on
    the command line. These end up as environment variables in the
	shell script.

  - Compilation flags that can only be decided when we run
    configure (this is currently just whether we support
    -Wl,--export-dynamic) are appended to the configure-time LDFLAGS
    environment variable.

  - At the end of the configure script, these environment variables
    are spliced into Makefile.in to fill out the corresponding
	@<varname>@ entries.

  - When running make, the user might again override compilation
    flags. These will get appended to the flags found so far.

As a concrete example:

  mkdir build
  cd build
  ../configure CXXFLAGS='-O3'
  make CXXFLAGS='-O0'

will result in c++ compile commands that look like this:

  g++ -MMD -MP \
      -DPREFIX=\"/usr/local\" -Wall -Wno-unused -g -O2 -std=c++11 \
      -O3 \
      -O0 \
      -I. -I.. -I../fesvr -I../riscv -I../dummy_rocc -I../softfloat \
      -I../spike_main -fPIC -c ../fesvr/elfloader.cc

(I've added some newlines to wrap the long line).

Note that we have the base flags from Makefile.in (called
$(default-CXXFLAGS) there) first. Then we have the -O3 from the
configure command. Finally we have the -O0 from the Make command line.

And I can finally run "make CXXFLAGS='-O0 -g3'". Phew!

[1] https://www.gnu.org/software/automake/manual/html_node/Flag-Variables-Ordering.html

1 files changed, 36 insertions, 9 deletions

diff --git a/Makefile.in b/Makefile.in
index f3bfb3f..154a6fd 100644
--- a/Makefile.in
+++ b/Makefile.in
@@ -77,25 +77,52 @@ VPATH := $(addprefix $(src_dir)/, $(sprojs_enabled))
 # C++ compiler
 #  - CPPFLAGS : flags for the preprocessor (eg. -I,-D)
 #  - CXXFLAGS : flags for C++ compiler (eg. -Wall,-g,-O3)
+#
+# To allow a user to specify CFLAGS or similar as part of the Make
+# command, we also have mcpps-CFLAGS etc. with stuff that shouldn't be
+# lost in such a case.
+#
+# The order of precedence (highest to lowest) is then:
+#
+#    - Specified as part of Make command line
+#    - Specified as part of running configure
+#    - Specified here (default-CFLAGS)
+#
+# These all appear on the command line, from lowest precedence to
+# highest.
+
+default-CFLAGS   := -DPREFIX=\"$(prefix)\" -Wall -Wno-unused -g -O2
+default-CXXFLAGS := $(default-CFLAGS) -std=c++11
+
+mcppbs-CPPFLAGS := @CPPFLAGS@
+mcppbs-CFLAGS   := $(default-CFLAGS) @CFLAGS@
+mcppbs-CXXFLAGS := $(default-CXXFLAGS) @CXXFLAGS@
 
 CC            := @CC@
 CXX           := @CXX@
-CFLAGS        += @CFLAGS@ -DPREFIX=\"$(prefix)\"
-CPPFLAGS      += @CPPFLAGS@
-CXXFLAGS      += @CXXFLAGS@ -DPREFIX=\"$(prefix)\"
-COMPILE       := $(CXX) -MMD -MP $(CPPFLAGS) $(CXXFLAGS) \
-                 $(sprojs_include)
-COMPILE_C     := $(CC) -MMD -MP $(CPPFLAGS) $(CFLAGS) \
-                 $(sprojs_include)
+
+# These are the flags actually used for a C++ compile or a C compile.
+# The language-specific flags come after the preprocessor flags, but
+# user-supplied flags always take precedence.
+all-cxx-flags := \
+  $(mcppbs-CPPFLAGS) $(mcppbs-CXXFLAGS) $(CPPFLAGS) $(CXXFLAGS)
+all-c-flags := \
+  $(mcppbs-CPPFLAGS) $(mcppbs-CFLAGS) $(CPPFLAGS) $(CFLAGS)
+
+COMPILE       := $(CXX) -MMD -MP $(all-cxx-flags) $(sprojs_include)
+COMPILE_C     := $(CC) -MMD -MP $(all-c-flags) $(sprojs_include)
+
 # Linker
 #  - LDFLAGS : Flags for the linker (eg. -L)
 #  - LIBS    : Library flags (eg. -l)
 
+mcppbs-LDFLAGS := @LDFLAGS@
+all-link-flags := $(mcppbs-LDFLAGS) $(LDFLAGS)
+
 comma := ,
 LD            := $(CXX)
-LDFLAGS       := @LDFLAGS@
 LIBS          := @LIBS@
-LINK          := $(LD) -L. $(LDFLAGS) -Wl,-rpath,$(install_libs_dir) $(patsubst -L%,-Wl$(comma)-rpath$(comma)%,$(filter -L%,$(LDFLAGS)))
+LINK          := $(LD) -L. $(all-link-flags) -Wl,-rpath,$(install_libs_dir) $(patsubst -L%,-Wl$(comma)-rpath$(comma)%,$(filter -L%,$(LDFLAGS)))
 
 # Library creation