Cmake update and some fixes also

CMake: be more robust and also tell us what we are building for
Clang: fix build errors on mingw-w64. 
Ninja: add build support for mingw-w64

@walzer This should be a good start to making a first installable package for cocos2d-x
We have /lib/libcocos2d.a
Now we just need includes in /include/