configure.ac (gcc_gxx_include_dir_add_sysroot): Set it to 1 only when --with-gxx-include-dir is also specified.

	* configure.ac (gcc_gxx_include_dir_add_sysroot): Set it to 1 only
	when --with-gxx-include-dir is also specified.
	* configure: Regenerate.

From-SVN: r260108