Use substituted GDCFLAGS

Use the substituted value for GCDFLAGS instead of hardcoding $(CFLAGS) so
that the subdir configure scripts use the configured value.

	* configure.ac (GDCFLAGS): Set default from ${CFLAGS}.
	* configure: Regenerate.
	* Makefile.in (GDCFLAGS): Substitute @GDCFLAGS@.