Unable to get include directory in dependent

In the build.rs is the instruction println!("cargo:include={}", p.display());, that is supposed to hand the include directory to a dependent crate. But I can't access it. I think there needs to be a links = "nettle" statement in the Cargo.toml to have the path available as the env variable DEP_NETTLE_INCLUDE.

Also this statement is in a loop and I am not able to store multiple values in one environment variable.