libx11: update to 1.6.8

libx11: update to 1.6.8

Fix the build when the host doesn't have libc.a installed by reverting an
upstream commit, and cherry-pick a build fix from master.

Drop a redundant sed, the expression it removes doesn't exist anymore.

Move most patches to libx11.inc so that they're applied to both libx11 and libx11-diet.

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>