CMake - Append new C flags when adding defines instead of overriding.

When using cocos2d-x as a subproject in a cmake file, existing compiler flags are overwritten. On Windows, the build might fail if debug flags are not uniform across the build (such as /MDd vs /MD).

See http://voices.canonical.com/jussi.pakkanen/2013/03/26/a-list-of-common-cmake-antipatterns/