Avoid overriding files shipped with CMake (CMP0017)

We ship our own versions of the following CMake-provided files:

• Compiler/GNU.cmake
• Compiler/VISUAL-ASM.cmake
• Platform/Windows.cmake
• Platform/Windows-MSVC.cmake

This requires the OLD setting for policy CMP0017, and our current copies of these files also don't work with CMake 3.10.
Ideally, we can remove those overrides completely (if they're not needed in 3.2.1), or at least disable them for newer CMake versions.