/usr/X11R6 doesn't exist on OS X per default (anymore). Therefore add
PATHS to all X11 detection modules pointing to the Xquarz installation
directory in /opt/X11.
For FindX11 it was also necessary to ensure that the frameworks are
searched as last (after PATHS) otherwise it could happen that X11 headers
of a framework (e.g. Tk.framework) were used.
FIND_PACKAGE_HANDLE_STANDARD_ARGS handles REQUIRE only correct if
called with the same package name as find_package was called with.
The returned xxxx_FOUND is still uppercase.
