CMAKE_HOST_SYSTEM
From Get docs
Cmake/docs/3.21/variable/cmake host system
CMAKE_HOST_SYSTEM
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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https://cmake.org/cmake/help/v3.21/variable/CMAKE_HOST_SYSTEM.html