The external toolchain for cross-compiling, if supported.
Some compiler toolchains do not ship their own auxiliary 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 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.
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Licensed under the BSD 3-clause License.
https://cmake.org/cmake/help/v3.19/variable/CMAKE_LANG_COMPILER_EXTERNAL_TOOLCHAIN.html