Made cmake use the built in find X11 scripts by default on OS X.

dlib/CMakeLists.txt

@@ -148,6 +148,11 @@ if (NOT TARGET dlib)
          set (dlib_needed_libraries ${pthreadlib})
 
          if (NOT DLIB_NO_GUI_SUPPORT)
+            find_package(X11 QUIET)
+            if (X11_FOUND)
+               include_directories(${X11_INCLUDE_DIR})
+               set (dlib_needed_libraries ${dlib_needed_libraries} ${X11_LIBRARIES})
+            else()
                find_library(xlib X11)
                # Make sure X11 is in the include path.  Note that we look for
                # Xlocale.h rather than Xlib.h because it avoids finding a partial
@@ -162,7 +167,10 @@ if (NOT TARGET dlib)
                   get_filename_component(x11_path ${xlib_path} PATH CACHE)
                   include_directories(${x11_path})
                   set(dlib_needed_libraries ${dlib_needed_libraries} ${xlib} )
-            else()
+                  set(X11_FOUND 1)
+               endif()
+            endif()
+            if (NOT X11_FOUND)
                message(" *****************************************************************************")
                message(" *** DLIB GUI SUPPORT DISABLED BECAUSE X11 DEVELOPMENT LIBRARIES NOT FOUND ***")
                message(" *** Make sure XQuartz is installed if you want GUI support.               ***")