... can cargo build things that aren't rust? If yes, that's really cool. If no, then it's not really in the same problem domain.

No it can't.

It can build a Rust program (build.rs) which builds things that aren't Rust, but that's an entirely different use case (building non-Rust library to use inside of Rust programs).

There's GprBuild (Ada tool) that can build C (not sure about C++). It also has more elaborate configuration structure, but I didn't use it extensively to tell what exactly and how exactly does it do it. In combination with Alire it can also manage dependencies Cargo-style.