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
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
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.