Get rid of unnecessary internal MagnumObjects OBJECT library.

It was needed for running some tests with graceful assert, but is not
needed anymore. This should also fix issues with CMake-generated XCode 5
project. Also fixed a few oddities:

 * Setting CMAKE_SHARED_LIBRARY_CXX_FLAGS only if it's really needed.
 * Don't add -DGLLoadGen_EXPORTS when building Math library, as it is
   not needed at all.