Allow for /usr/X11R6 directory or symlink

If a system has a /usr/X11R6 directory or symlink, the .pro files do not move the /usr/X11R6 include to the end, causing build failures. This commit fixes libmyth.pro to relocate this directory, as well as /usr/local and /usr/local/include.

While here, add the missing assignment of POSTINC back to INCLUDEPATH.