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, 4 insertions, 3 deletions

diff --git a/configure.ac b/configure.ac
index a6f52d8..49e2e0d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -43,7 +43,10 @@ AC_CONFIG_AUX_DIR([scripts])
 AC_CANONICAL_BUILD
 AC_CANONICAL_HOST
 
+m4_include(ax_require_defined.m4)
+m4_include(ax_append_flag.m4)
 m4_include(ax_check_link_flag.m4)
+m4_include(ax_append_link_flags.m4)
 
 #-------------------------------------------------------------------------
 # Checks for programs
@@ -84,9 +87,7 @@ AC_CHECK_TYPE([__int128_t], AC_SUBST([HAVE_INT128],[yes]))
 # Default compiler flags
 #-------------------------------------------------------------------------
 
-AC_SUBST([CFLAGS],  ["-Wall -Wno-unused -g -O2"])
-AC_SUBST([CXXFLAGS],["-Wall -Wno-unused -g -O2 -std=c++11"])
-AX_CHECK_LINK_FLAG([-Wl,--export-dynamic], [LDFLAGS="-Wl,--export-dynamic"])
+AX_APPEND_LINK_FLAGS([-Wl,--export-dynamic])
 
 #-------------------------------------------------------------------------
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