Suppress harmless clang 3.3 warning about unused wxMessageOutputBest field.

Clang detects that a class member is unused (under non-Windows systems) which
is quite impressive but not really useful in this particular case, so suppress
this warning by "using" it.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74519 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/msgout.cpp

@@ -126,6 +126,8 @@ void wxMessageOutputBest::Output(const wxString& str)
 
     ::MessageBox(NULL, str.t_str(), title.t_str(), MB_ICONINFORMATION | MB_OK);
 #else // !__WINDOWS__
+    wxUnusedVar(m_flags);
+
     // TODO: use the native message box for the other ports too
     wxMessageOutputStderr::Output(str);
 #endif // __WINDOWS__/!__WINDOWS__