configure: Don't override user -g or -O options for debug builds

configure: Don't override user -g or -O options for debug builds

Principle of least surprise: --enable-debug should enable debugging.
Ages ago, Mesa's build system only added -g in dri-debug builds (yay for
the static Makefiles).  If you forgot to change it (or wrap the build
with custom scripts), you would often be disappointed when trying to gdb
Mesa bugs.  New developers, that may not yet have custom scripts, will
have this same issue.

I think we should enable experienced developers to do what they want,
and make things easier for new developers.  I already pass '-ggdb3 -O1'
or '-ggdb3 -Og' for CFLAGS, and I don't want configure to change them
for me.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>