Define wxHAS_NATIVE_ANIMATIONCTRL

This allows to easily test for the native control existence and also
whether wxGenericAnimationCtrl is really different from wxAnimationCtrl.

It also allows to avoid explicit check for wxGTK in common code.

include/wx/animate.h

@@ -167,6 +167,8 @@ private:
 
 #if defined(__WXGTK20__)
     #include "wx/gtk/animate.h"
+
+    #define wxHAS_NATIVE_ANIMATIONCTRL
 #else
     class WXDLLIMPEXP_ADV wxAnimationCtrl : public wxGenericAnimationCtrl
     {