CMAKE_HOST_UNIX

From Get docs
Cmake/docs/3.21/variable/cmake host unix


CMAKE_HOST_UNIX

True for UNIX and UNIX like operating systems.

Set to true when the host system is UNIX or UNIX like (i.e. APPLE and CYGWIN).

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Licensed under the BSD 3-clause License.
https://cmake.org/cmake/help/v3.21/variable/CMAKE_HOST_UNIX.html