* introduce config variable USE_PREBUILT_LIBS (default: ON). When OFF, cmake will not use prebuilt libs, only try find system installed libraries.
* FindGLFW3.cmake now search our prebuilt library (if USE_PREBULT_LIBS)
* FindFreetype.cmake also
This and next changes should simplify our CMakeLists.txt files and USE_PREBUILT_LIBS config allow external package developers to integrate cocos to their packaging system.