CMake: Fix /W4 override for CMake 3.15+
CMake: Fix /W4 override for CMake 3.15+

Starting from CMake 3.15 we can set a policy that removes the default
/W3 flag from compilation options, making /W4 overrides we use
compatible with ninja.

zeux/meshoptimizer,zeux/meshoptimizer,zeux/meshoptimizer