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
-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
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.