By default, the project() CMake command defaults to C and C++. [1]
Therefore, CMake might perform tests for both C and C++ compilers as
part of the configuration phase.
However, this has the consequence of the configuration phase to fail if
the system does not have a C++ toolchain installed, even if C++ is not
really used by the top-level project under the default settings.
Some configurations might still require a C++ toolchain, so
enable_language is selectively called under such circumstances.
[1]: https://cmake.org/cmake/help/latest/command/project.html
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