Let --enable-debug determine whether optimization should be disabled,
which works also for debug=t due to the setup in gbuild.mk.

Also clean up uses of gb_COMPILEROPTFLAGS: this is now always set to
optimization settings, and gb_COMPILERNOOPTFLAGS used in debug case,
which should not make a difference because the only actual difference
was that on MSVC the debug OPTFLAGS were empty, but -Od is documented as
the default for this.