Allow for existence of X11R6 directory/symlink

When a system has a /usr/X11R6 directory, this directory is used as an include. Some system have this as a symlink to /usr/local. The code to move generic includes to the end of the list of includes doesn't catch this, leading to incorrect includes and build failures.

Add code to the .pro files to move this directory to the end, following the example used with /usr/include and /usr/local/include. While here, add the reassignment of INCLUDEPATH += $$POSTINC in libmyth.pro, which had been missed when the code had been added. It seems to work without it (has done for years!), but this is what the code was meant to do.

Suggestion - this is the correct thing to do everywhere, not just where it causes problems. Should this code be added to all the .pro files, or done universally in the targetfix.pro or other included files?