message: Add Linux support for opening Wiki link
diff: @@ -245,6 +245,8 @@ void MainMenuState::btnHelpClicked()
  system("start https://wiki.outpost2.net/doku.php?id=outposthd:how_to_play");
  #elif defined(__APPLE__)
  system("open https://wiki.outpost2.net/doku.php?id=outposthd:how_to_play");
  +#elif defined(__linux__)
  + system("xdg-open https://wiki.outpost2.net/doku.php?id=outposthd:how_to_play");
  #else
  //#error Open a web page support on the current platform not implemented.
  #pragma message( "Open a web page support on the current platform not implemented." )
  