CMAKE_VS_PLATFORM_TOOLSET_HOST_ARCHITECTURE
Visual Studio preferred tool architecture.
The Visual Studio Generators for VS 2013 and above support optional selection of a 64-bit toolchain on 64-bit hosts by specifying a host=x64 value in the CMAKE_GENERATOR_TOOLSET option. CMake provides the selected toolchain architecture preference in this variable (either x64 or empty).
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https://cmake.org/cmake/help/v3.9/variable/CMAKE_VS_PLATFORM_TOOLSET_HOST_ARCHITECTURE.html