Native build system toolset name specified by user.

Some CMake generators support a toolset name to be given to the native build system to choose a compiler. If the user specifies a toolset name (e.g. via the cmake(1) -T 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_TOOLSET. Once a given build tree has been initialized with a particular value for this variable, changing the value has undefined behavior.