Composite Name of OS CMake is being run on.
This variable is the composite of CMAKE_HOST_SYSTEM_NAME and CMAKE_HOST_SYSTEM_VERSION, e.g. ${CMAKE_HOST_SYSTEM_NAME}-${CMAKE_HOST_SYSTEM_VERSION}. If CMAKE_HOST_SYSTEM_VERSION is not set, then this variable is the same as CMAKE_HOST_SYSTEM_NAME.
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Licensed under the BSD 3-clause License.
https://cmake.org/cmake/help/v3.19/variable/CMAKE_HOST_SYSTEM.html