configure.ac (gcc_gxx_include_dir): Use $(libsubdir_to_prefix).

gcc/
	* configure.ac (gcc_gxx_include_dir): Use $(libsubdir_to_prefix).
	(gcc_tooldir): Likewise.
	* configure: Regenerate.
	* Makefile.in (libsubdir_to_prefix): New variable, based on the
	old configure.ac gcc_tooldir setting.
	(prefix_to_exec_prefix): New variable.
	(DRIVER_DEFINES): Use $(libsubdir_to_prefix)$(prefix_to_exec_prefix)
	rather than $(unlibsubdir)/../ to derive TOOLDIR_BASE_PREFIX.

From-SVN: r126225