CMake modules required for building Lean

The files GetGitRevisionDescription.cmake, GetGitRevisionDescription.cmake.in and FindWindowsSDK.cmake are part of the cmake-modules project by Rylie Pavlik. They are licensed under the Boost Software License, Version 1.0.

The modules should be updated when a new version of the Windows SDK is released. To do so, obtain an up-to-date checkout of the cmake-modules project from https://github.com/rpavlik/cmake-modules and run the following command from the root of the cmake-modules repository:

./update-modules.sh /path-to-lean4-repo/src/cmake/Modules