bug#79648: configure.ac sets user-defined CFLAGS

Hello list,

The CFLAGS variable should not be set in configure.ac. Of course, it
is fine to temporarily set it to a different value to test
compilation, but it should not set "defaults" in CFLAGS, since that
overrides whatever the user sets in the environment.

For example, I compiled with CFLAGS='-O0 g3' to debug a (different)
bug in Guile with gdb, and I did not have any access to macros. After
a lot of head scratching I resorted to asking for help and
https://sourceware.org/bugzilla/show_bug.cgi?id=32381 was pointed out
to me. In short, Guile injected -flto which enables LTO and gdb cannot
trace line numbers correctly.

Instead, configure.ac should be setting defaults in AM_CFLAGS. I will
send a patch moving -flto and -Werror=array-bounds from CFLAGS to
AM_CFLAGS.

Regards,
Nikolaos Chatzikonstantinou