rebuild openssl-sys if the underlying openssl has changed

    let target = env::var("TARGET").unwrap();

    let (lib_dirs, include_dir) = find_openssl(&target);

    if let Some(printable_include) = include_dir.to_str() {

        println!("cargo:rerun-if-changed={}", printable_include);

    }

    if !lib_dirs.iter().all(|p| Path::new(p).exists()) {

        panic!("OpenSSL library directory does not exist: {:?}", lib_dirs);