show help message when pkg-config is missing

        return

    }

    let lib = pkg_config::Config::new()

    let lib = match pkg_config::Config::new()

        .print_system_libs(false)

        .find("openssl")

        .unwrap();

        .find("openssl") {

        Ok(lib) => lib,

        Err(e) => {

            println!("run pkg_config fail: {:?}", e);

            return;

        }

    };

    validate_headers(&lib.include_paths);