automake: Remove OPT_FLAGS.

automake: Remove OPT_FLAGS.

If you want to change your compiler arguments, just set CFLAGS/CXXFLAGS.
Having Mesa have this separate variable is a great way to have your arguments
not thoroughly propagated to all compiler invocations.

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