Fix building with OS X < 10.9

wxNotificationMessage changes in bf5e403 included a 10.9+ property runtime
check but was missing a compile guard.

Closes https://github.com/wxWidgets/wxWidgets/pull/220

src/osx/cocoa/notifmsg.mm

@@ -119,9 +119,11 @@ public:
     
     virtual void SetIcon(const wxIcon& icon) wxOVERRIDE
     {
+#if MAC_OS_X_VERSION_MAX_ALLOWED >= MAC_OS_X_VERSION_10_9
         // Additional icon in the notification is only supported on OS X 10.9+
         if ([NSUserNotification instancesRespondToSelector:@selector(setContentImage:)])
             m_notif.contentImage = icon.GetNSImage();
+#endif
     }
     
     virtual bool AddAction(wxWindowID actionid, const wxString &label)