CMAKE_GENERATOR_PLATFORM
From Get docs
Cmake/docs/3.21/envvar/cmake generator platform
CMAKE_GENERATOR_PLATFORM
New in version 3.15.
This is a CMake Environment Variable. Its initial value is taken from the calling process environment.
Default value for CMAKE_GENERATOR_PLATFORM if no Cache entry is present and no value is specified by cmake(1) -A
option. This value is only applied if CMAKE_GENERATOR is set.
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https://cmake.org/cmake/help/v3.21/envvar/CMAKE_GENERATOR_PLATFORM.html