CMAKE_HOST_SYSTEM_NAME

Name of the OS CMake is running on.

On systems that have the uname command, this variable is set to the output of uname -s. Linux, Windows, and Darwin for OS X are the values found on the big three operating systems.

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