Only include/build pnghand if the user builds the png library (avoids multiple build errors)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@29262 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/mac/carbon/pnghand.h

@@ -9,12 +9,16 @@
 // Licence:     wxWindows licence
 /////////////////////////////////////////////////////////////////////////////
 
+#ifndef _WX_PNGHAND_H_
+#define _WX_PNGHAND_H_
+
 #if defined(__GNUG__) && !defined(NO_GCC_PRAGMA)
 #pragma interface "pnghand.h"
 #endif
 
-#ifndef _WX_PNGHAND_H_
-#define _WX_PNGHAND_H_
+#include "wx/defs.h"
+
+#if wxUSE_PNGLIB
 
 class WXDLLEXPORT wxPNGFileHandler: public wxBitmapHandler
 {
@@ -32,6 +36,8 @@ public:
   virtual bool SaveFile(const wxBitmap *bitmap, const wxString& name, int type, const wxPalette *palette = NULL);
 };
 
+#endif //wxUSE_LIBPNG
+
 #endif
   // _WX_PNGHAND_H_