Updates for wx3

A long way to go yet, but many OSX issues fixed including
conversion of Audio Unit effects.

src/prefs/LibraryPrefs.cpp

@@ -142,7 +142,8 @@ void LibraryPrefs::PopulateOrExchange(ShuttleGui & S)
          bfnd->Enable(FALSE);
 #else
          // fix compilation warnings about unused variables
-         bfnd, bdwn;
+         wxUnusedVar(bfnd);
+         wxUnusedVar(bdwn);
 #endif
       }
       S.EndTwoColumn();