Now build system looks more maintanable.
I test with mac, going to test with other platforms.
* All prebuilt libs config moved to
cmake/Modules/CocosUsePrebuiltLibs.cmake
* Wrap plain find_package to our cocos_find_package. It will not try to
call find_package if USE_PREBUILT_LIBS and <pkg>_FOUND set to true.
* Other small fixes.
* 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.