MFH: r477818

x11/libX11: Update to 1.6.6

This is a security update

Changelog:
https://lists.x.org/archives/xorg-announce/2018-August/002916.html

Note: I had to hand-merge Makefile a little because of differences in

portrevision.

Security: fe99d3ca-a63a-11e8-a7c6-54e1ad3d6335

Approved by: ports-secteam (miwi)