Replaced deprecated `DetermineVSServicePack`macro with recommended
`CMAKE_CXX_COMPILER_VERSION`.
Deleted custom `OSVersion` detection. The windows-specific code is
highly outdated, and on other systems simply returns `CMAKE_SYSTEM`.
We will get values like `windows-10.0.17763`, but this is preferable
to `unknownwin`, and saves us needing to maintain a separate cmake file.