Work-around preprocessor issues on Windows.

I hate windows.h.
Fix linker issues on Windows.

MagnumSceneGraphTestLib didn't have MagnumSceneGraph[Objects]_EXPORT
defined, thus failing at link time, similar issue with
MagnumMeshToolsTestLib.
Explicitly link MagnumMathTestLib to Corrade's Utility library.

Would be linked later along TestSuite, but the linker complained now.