The external toolchain for cross-compiling, if supported.

Some compiler toolchains do not ship their own auxilliary utilities such as archivers and linkers. The compiler driver may support a command-line argument to specify the location of such tools. CMAKE_<LANG>_COMPILER_EXTERNAL_TOOLCHAIN may be set to a path to a path to the external toolchain and will be passed to the compiler driver if supported.

This variable may only be set in a toolchain file specified by the CMAKE_TOOLCHAIN_FILE variable.