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2020-02-27Allow overriding CFLAGS and similar when buildingRupert Swarbrick1-0/+44
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