CMAKE_SYSTEM
Composite name of operating system CMake is compiling for.
This variable is the composite of CMAKE_SYSTEM_NAME and CMAKE_SYSTEM_VERSION, e.g. ${CMAKE_SYSTEM_NAME}-${CMAKE_SYSTEM_VERSION}. If CMAKE_SYSTEM_VERSION is not set, then this variable is the same as CMAKE_SYSTEM_NAME.
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Licensed under the BSD 3-clause License.
https://cmake.org/cmake/help/v3.5/variable/CMAKE_SYSTEM.html