Fix for "light" cross-compile (eg. X32 or 32-bit, on a 64-bit host).
We must preserve CFLAGS but if we put -O0 in its tail, we override
any -O2 that come before it.

Also, ditch the AC_USE_SYSTEM_EXTENSIONS macro. It's not compatible
with AX_PROG_CC_MPI at all. We currently handle such extensions in
os.h so we don't really need it right now.