CMAKE_CACHE_MAJOR_VERSION

Major version of CMake used to create the CMakeCache.txt file

This stores the major version of CMake used to write a CMake cache file. It is only different when a different version of CMake is run on a previously created cache file.

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Licensed under the BSD 3-clause License.
https://cmake.org/cmake/help/v3.5/variable/CMAKE_CACHE_MAJOR_VERSION.html