Reapply libiberty: Pass --plugin to AR and RANLIB

Reapply the patch to detect GCC LTO plugin used for libiberty build to
support LTO build in libiberty.

	* Makefile.in (AR): Add @AR_PLUGIN_OPTION@
	(RANLIB): Add @RANLIB_PLUGIN_OPTION@.
	(configure_deps): Depend on ../config/gcc-plugin.m4.
	* aclocal.m4: Include ../config/gcc-plugin.m4.
	* configure.ac: AC_SUBST AR_PLUGIN_OPTION and
	RANLIB_PLUGIN_OPTION.
	* configure: Regenerate.