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, 44 insertions, 0 deletions

diff --git a/ax_append_link_flags.m4 b/ax_append_link_flags.m4
new file mode 100644
index 0000000..99b9fa5
--- /dev/null
+++ b/ax_append_link_flags.m4
@@ -0,0 +1,44 @@
+# ===========================================================================
+#   https://www.gnu.org/software/autoconf-archive/ax_append_link_flags.html
+# ===========================================================================
+#
+# SYNOPSIS
+#
+#   AX_APPEND_LINK_FLAGS([FLAG1 FLAG2 ...], [FLAGS-VARIABLE], [EXTRA-FLAGS], [INPUT])
+#
+# DESCRIPTION
+#
+#   For every FLAG1, FLAG2 it is checked whether the linker works with the
+#   flag.  If it does, the flag is added FLAGS-VARIABLE
+#
+#   If FLAGS-VARIABLE is not specified, the linker's flags (LDFLAGS) is
+#   used. During the check the flag is always added to the linker's flags.
+#
+#   If EXTRA-FLAGS is defined, it is added to the linker's default flags
+#   when the check is done.  The check is thus made with the flags: "LDFLAGS
+#   EXTRA-FLAGS FLAG".  This can for example be used to force the linker to
+#   issue an error when a bad flag is given.
+#
+#   INPUT gives an alternative input source to AC_COMPILE_IFELSE.
+#
+#   NOTE: This macro depends on the AX_APPEND_FLAG and AX_CHECK_LINK_FLAG.
+#   Please keep this macro in sync with AX_APPEND_COMPILE_FLAGS.
+#
+# LICENSE
+#
+#   Copyright (c) 2011 Maarten Bosmans <mkbosmans@gmail.com>
+#
+#   Copying and distribution of this file, with or without modification, are
+#   permitted in any medium without royalty provided the copyright notice
+#   and this notice are preserved.  This file is offered as-is, without any
+#   warranty.
+
+#serial 7
+
+AC_DEFUN([AX_APPEND_LINK_FLAGS],
+[AX_REQUIRE_DEFINED([AX_CHECK_LINK_FLAG])
+AX_REQUIRE_DEFINED([AX_APPEND_FLAG])
+for flag in $1; do
+  AX_CHECK_LINK_FLAG([$flag], [AX_APPEND_FLAG([$flag], [m4_default([$2], [LDFLAGS])])], [], [$3], [$4])
+done
+])dnl AX_APPEND_LINK_FLAGS