cocoa: avoid another 10.6-only method

Closes PR 11440

collects/mred/private/wx/cocoa/filedialog.rkt

@@ -74,7 +74,9 @@
            ;; all other eventspaces and threads. It would be nice to improve
            ;; on this, but it's good enough.
            (atomically
-            (let ([front (get-front)])
+            (let ([front (get-front)]
+                  [parent (and (version-10.6-or-later?)
+                               parent)])
               (when parent
                 (tellv ns beginSheetModalForWindow: (send parent get-cocoa-window)
                        completionHandler: #f))

collects/mred/private/wx/cocoa/menu.rkt

@@ -63,7 +63,7 @@
       (create-menu "menu")
       (let ([b (box #f)])
         (set! popup-box b)
-        (if #t ;; use the 10.5 code, for now
+        (if (not (version-10.6-or-later?))
             ;; For 10.5 and earlier:
             (let ([p (tell #:type _NSPoint v
                            convertPoint: #:type _NSPoint (make-NSPoint x y)

collects/mred/private/wx/cocoa/utils.rkt

@@ -21,7 +21,8 @@
               clean-menu-label
               ->wxb
               ->wx
-              old-cocoa?)
+              old-cocoa?
+              version-10.6-or-later?)
  define-mz)
 
 (define cocoa-lib (ffi-lib (format "/System/Library/Frameworks/Cocoa.framework/Cocoa")))
@@ -79,6 +80,10 @@
   (and wxb
        (weak-box-value wxb)))
 
-;; FIXME: need a better test:
-(define old-cocoa? (equal? (path->string (system-library-subpath #f))
-                           "ppc-macosx"))
+(define-appkit NSAppKitVersionNumber _double)
+
+(define old-cocoa? 
+  ; earlier than 10.5?
+  (NSAppKitVersionNumber . < . 949))
+(define (version-10.6-or-later?)
+  (NSAppKitVersionNumber . >= . 1038))