don't emit rerun-if-changed unless the path exists and is readable

    // dir has changed since the last build. However, this causes a rebuild

    // every time when vendoring so we disable it.

    #[cfg(not(feature = "vendored"))]

    if let Some(printable_include) = include_dir.join("openssl").to_str() {

    let potential_path = include_dir.join("openssl");

    if potential_path.exists() {

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

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

        }

    }

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

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