Improve Mac wx3.1.1 compatibility

1. Qualify hash as std::hash.
2. Ensure we have a header for the function 'free'

lib-src/FileDialog/mac/FileDialogPrivate.mm

@@ -49,6 +49,8 @@
 // implementation
 // ============================================================================
 
+#define NSObject wxOSX_10_6_AND_LATER( x ) x
+
 @interface OSPanelDelegate : NSObject wxOSX_10_6_AND_LATER(<NSOpenSavePanelDelegate>)
 {
     FileDialog* _dialog;