/// $PackageInfo: require_system("darwin") gnutls pkg-config
/// $PackageInfo: require_system("ubuntu" "1.0" "13.10") libgcrypt11-dev
/// $PackageInfo: require_system("ubuntu" "14.04") gnutls-bin libgnutls-dev pkg-config
/// $PackageInfo: require_system("darwin") gnutls pkg-config
/// $PackageInfo: require_system("ubuntu" "1.0" "13.10") libgcrypt11-dev
/// $PackageInfo: require_system("ubuntu" "14.04") gnutls-bin libgnutls-dev pkg-config