acinclude.m4 (GCC_GLIBC_VERSION_GTE_IFELSE): New configure macro.

	* acinclude.m4 (GCC_GLIBC_VERSION_GTE_IFELSE): New configure
	macro.
	* configure.ac: Determine target_header_dir earlier.
	(--with-glibc-version): New configure option.
	Use GCC_GLIBC_VERSION_GTE_IFELSE in enable_gnu_unique_object,
	gcc_cv_libc_provides_ssp and gcc_cv_target_ldbl128 tests.
	* configure: Regenerate.
	* doc/install.texi (--enable-gnu-unique-object): Don't refer to
	native toolchains for default.
	(--with-glibc-version): Document.

From-SVN: r204841