Sync config/warnings.m4 with GCC

Sync config/warnings.m4 with GCC

config/

	Sync with GCC
	2015-05-27  Jason Merrill  <jason@redhat.com>

	PR bootstrap/66304
	* warnings.m4 (ACX_PROG_CXX_WARNING_OPTS)
	(ACX_PROG_CXX_WARNINGS_ARE_ERRORS)
	(ACX_PROG_CXX_WARNING_ALMOST_PEDANTIC): New.
	(ACX_PROG_CC_WARNING_OPTS, ACX_PROG_CC_WARNING_ALMOST_PEDANTIC)
	(ACX_PROG_CC_WARNINGS_ARE_ERRORS): Push into C language context.

libdecnumber/

	* configure: Regenerated.

libiberty/

	* configure: Regenerated.

opcodes/

	* configure: Regenerated.