Made cmake's "can't find X11" error message on OS X provide a link

to download XQuartz.

dlib/CMakeLists.txt

@@ -155,7 +155,8 @@ if (NOT TARGET dlib)
             else()
                message(" *****************************************************************************")
                message(" *** DLIB GUI SUPPORT DISABLED BECAUSE X11 DEVELOPMENT LIBRARIES NOT FOUND ***")
-               message(" *** Make sure libx11-dev is installed if you want GUI support             ***")
+               message(" *** Make sure XQuartz is installed if you want GUI support.               ***")
+               message(" *** You can download XQuartz from: http://xquartz.macosforge.org/landing/ ***")
                message(" *****************************************************************************")
                set(DLIB_NO_GUI_SUPPORT ON CACHE STRING ${DLIB_NO_GUI_SUPPORT_STR} FORCE )
             endif()