configure.ac: Fix CFLAGS settings.

configure.ac: Fix CFLAGS settings.

Don't override/overwrite CFLAGS in configure.ac, but rather amend it
with (currently) "-Wall -Wextra -fvisibility=hidden".

This properly allows users/packagers to do things like:

 ./configure (this will default to using "-g -O2" additionally)

 CFLAGS="-g -O2" ./configure (same as above)

 CFLAGS="" ./configure (no additional flags)

 CFLAGS="-g -O0" ./configure (disable optimization, e.g. for valgrind use)

 etc. etc.