CMAKE_SYSTEM

From Get docs
Cmake/docs/3.21/variable/cmake system


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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https://cmake.org/cmake/help/v3.21/variable/CMAKE_SYSTEM.html