Unfortunately, checking CARGO_CFG_TARGET_OS at runtime means that we always have to compile all branches of the build.rs script. This means that we always need to depend on pkg-config even in pure Rust builds. It might be annoying to add a build dependency when most setups probably will never need it.

@cberner if this is acceptable I will leave it like this. Otherwise I can look into some darker magic, because I can't think of an easy solution.

That sounds fine

If CI passes then I think this is good :)