message: 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.
diff: cmake_minimum_required(VERSION 3.0)
  -if (POLICY CMP0091)
  - cmake_policy(SET CMP0091 NEW)
  +if (CMAKE_VERSION VERSION_GREATER_EQUAL 3.15)
  + cmake_policy(SET CMP0091 NEW) # Enables use of MSVC_RUNTIME_LIBRARY
  + cmake_policy(SET CMP0092 NEW) # Enables clean /W4 override for MSVC
  endif()
  project(meshoptimizer VERSION 0.12 LANGUAGES CXX)
  