This fixes issues with building LibreSSL, which is required for the
NixOS tests: https://cmake.org/cmake/help/latest/release/3.19.html#id1
> CMake 3.19.0 compiles source files with the `LANGUAGE` property by
> passing an explicit language flag such as -x c. This is consistent
> with the property’s documented meaning that the source file is written
> in the specified language. However, it can break projects that were
> using the property only to cause the specified language’s compiler to
> be used. This has been reverted to restore behavior from CMake 3.18
> and below.
31dfbca2d7