It does support calling into other compilers and toolchains through build scripts and such. Take cc-rs[0] for example: this allows building C and C++ files natively without even calling an executable yourself.

In practice, I'd expect libraries to just call make/cmake/ninja for you, or (like openssl-sys) ask you to install the necessary libraries using your favourite package manager.

[0]: https://github.com/rust-lang/cc-rs