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