Disable libstdc++ dependency for libffi

Disable libstdc++ dependency for libffi

Disable AC_PROG_CXX and consequently a libstdc++ dependency for libffi,
introduced with upstream libffi commit 7d698125b1f0 ("Use the proper C++
compiler to run C++ tests").  This is only needed for the libffi test
suite, which we don't have to support in the GCC tree, as libffi is
maintained as a separate project.  The dependency causes a build failure
with the `powerpc64le-linux-gnu' target due to a circular dependency:

make[1]: Circular configure-target-libffi <- maybe-all-target-libstdc++-v3 dependency dropped.
make[1]: *** [configure-target-libffi] Error 1
make: *** [all] Error 2

due to a libgomp dependency for libstdc++ and then a libffi dependency
for libgomp, introduced with commit 998eb38b265d ("Use functional
parameters for data mappings in OpenACC child functions").

	/
	* Makefile.def (lang_env_dependencies): Disable `cxx' dependency
	for `libffi'.
	* Makefile.in: Regenerate.

	libffi/
	* configure.ac: Disable AC_PROG_CXX.
	* configure: Regenerate.
	* Makefile.in: Regenerate.
	* include/Makefile.in: Regenerate.
	* man/Makefile.in: Regenerate.
	* testsuite/Makefile.in: Regenerate.