message: Fix CLI::App clang5/6 warning wrt non-virtual dtor
diff: @@ -1190,7 +1190,7 @@ using App_p = std::unique_ptr<App>;
  /** To use, create a new `Program()` instance with `argc`, `argv`, and a help description. The templated
  * add_option methods make it easy to prepare options. Remember to call `.start` before starting your
  * program, so that the options can be evaluated and the help option doesn't accidentally run your program. */
  -class App {
  +class App final {
  friend Option;
  friend detail::AppFriend;
  