Generator-specific target platform name specified by user.
Some CMake generators support a target platform name to be given to the native build system to choose a compiler toolchain. If the user specifies a platform name (e.g. via the cmake(1)
-A option) the value will be available in this variable.
The value of this variable should never be modified by project code. A toolchain file specified by the CMAKE_TOOLCHAIN_FILE variable may initialize
CMAKE_GENERATOR_PLATFORM. Once a given build tree has been initialized with a particular value for this variable, changing the value has undefined behavior.