configure.ac: Remove -Werror addition to WARN_FLAGS.

	* configure.ac: Remove -Werror addition to WARN_FLAGS.  Add
	ACX_PROG_CC_WARNINGS_ARE_ERRORS and AC_ARG_VAR for GMPINC.
	* Makefile.am (AM_CPPFLAGS): Add $(GMPINC).
	(WERROR_FLAG): Remove.
	(AM_CXXFLAGS): Use $(WERROR) instead of $(WERROR_FLAG).
	* configure: Regenerated.
	* Makefile.in: Regenerated.

Co-Authored-By: Phil Muldoon <pmuldoon@redhat.com>

From-SVN: r216833