@eht16 commented on this pull request.
-#if ! GTK_CHECK_VERSION(2, 10, 0)
gtk_widget_hide(check_systray); -#endif
This is so me...you are completely right. Thanks for spotting, fixed.
Funny thing, this attempt to hide the option didn't work at all because in Geany itself we call `gtk_widget_show_all()` when creating the plugin preferences dialog and so it was shown again. Anyway, GTK2 is long time ago.