Fix Windows CMake compiler/OS detection.

Antonio Sánchezrequested to merge
cantonios/eigen:cmake_windows into master

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 The windows-specific code is highly outdated, and on other systems simply returns CMAKE_SYSTEM.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.

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