org-mouse: Make recently ported change compatible with Emacs < 29

* lisp/org-mouse.el (org-mouse-show-context-menu): Conditionally
restore old code path, falling back to double-click-time if
mouse-double-click-time isn't available.

This is a follow-up to the port of Emacs's 2fba71cf1.

Author: kyleam

lisp/org-mouse.el

@@ -211,7 +211,11 @@ this function is called.  Otherwise, the current major mode menu is used."
   (interactive "@e \nP")
   (if (and (= (event-click-count event) 1)
 	   (or (not mark-active)
-               (sit-for (/ (mouse-double-click-time) 1000.0))))
+               (sit-for
+                (/ (if (fboundp 'mouse-double-click-time) ; Emacs >= 29
+                       (mouse-double-click-time)
+                     double-click-time)
+                   1000.0))))
       (progn
 	(select-window (posn-window (event-start event)))
 	(when (not (org-mouse-mark-active))