Updates to the CMake build system for Mac OSX

src/osgPlugins/avfoundation/CMakeLists.txt

@@ -1,3 +1,8 @@
+# AVFoundation plugin only works with OSX/Cocoa (not X11 or Carbon)
+IF(NOT OSG_WINDOWING_SYSTEM STREQUAL "Cocoa")
+  MESSAGE(WARNING "Disabling AVFoundation plugin because it requires OSG_WINDOWING_SYSTEM to be Cocoa")
+  RETURN()
+ENDIF()
 
 SET(TARGET_SRC
     OSXAVFoundationVideo.mm