x11/libX11: Update to 1.6.8

Update x11/libX11 to 1.6.8

This is a farirly big update

Add a patch from upstream to fix a build issue when X headers aren't
installed in the default search path of the compiler.

Changelog:
https://lists.x.org/archives/xorg-announce/2019-June/002997.html

Obtained from: Graphics Team development repo

		https://github.com/FreeBSDDesktop/freebsd-ports/tree/feature/xorglibs

Sponsored by: B3 Init