metadeps lets you write pkg-config dependencies in Cargo.toml metadata, rather than programmatically in build.rs. This makes those dependencies declarative, so other tools can read them as well.

In your Cargo.toml, add the following to your [build-dependencies]:

metadeps = "1.1"

Then, to declare a dependency on testlib >= 1.2, and a conditional dependency on testdata >= 4.5, add the following section:

[package.metadata.pkg-config]
testlib = "1.2"
testdata = { version = "4.5", feature = "use-testdata" }

In your build.rs, add:

extern crate metadeps;

fn main() {
    metadeps::probe().unwrap();
}