Example result:

$ cat /usr/include/X11/X.h 
/* This file is auto-generated by autotools-multilib.eclass
 * as a multilib-friendly wrapper. For the original content,
 * please see the files that are #included below.
 */

#if defined(__i386__)
#	include <i686-pc-linux-gnu/X11/X.h>
#elif defined(__x86_64__) && !defined(__ILP32__)
#	include <x86_64-pc-linux-gnu/X11/X.h>
#else
#	error "No ABI matched, please report a bug to bugs.gentoo.org"
#endif

-- 
Best regards,
Michał Górny