/// $PackageInfo: require_system("darwin") pkg-config re2
/// $PackageInfo: require_system("debian" "8.0") libre2-dev pkg-config
/// $PackageInfo: require_system("ubuntu" "15.10") libre2-dev pkg-config
/// $PackageInfo: require_system("darwin") pkg-config re2
/// $PackageInfo: require_system("debian" "8.0") libre2-dev pkg-config
/// $PackageInfo: require_system("ubuntu" "15.10") libre2-dev pkg-config