configure.ac: Add --enable-build-poststage1-with-cxx.

configure.ac: Add --enable-build-poststage1-with-cxx.

./:	* configure.ac: Add --enable-build-poststage1-with-cxx.  If set,
	make C++ a boot_language.  Set and substitute
	POSTSTAGE1_CONFIGURE_FLAGS.
	* Makefile.tpl (POSTSTAGE1_CONFIGURE_FLAGS): New variable.
	(STAGE[+id+]_CONFIGURE_FLAGS): Add $(POSTSTAGE1_CONFIGURE_FLAGS).
	* configure, Makefile.in: Rebuild.
gcc/:
	* doc/install.texi (Configuration): Document
	--enable-build-poststage1-with-cxx.

From-SVN: r176480