rusty-cheddar examples suggest using build.rs to generate C headers into target/include/. Alternatively, a crate might ship checked-in headers under include/. If my top-level crate depends on a crate that does either, it seems that the headers don't show up under the top-level crate's target/.
How do I make the C headers that a dependency ships or generates during build show up under target/ so that C project can use them?