cast: suppress more warnings for 'ffs'

cast.hpp

@@ -3,13 +3,13 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/.
  *
- * Copyright 2011 Danny Robson <danny@nerdcruft.net>
+ * Copyright 2011-2018 Danny Robson <danny@nerdcruft.net>
  */
 
-#ifndef __UTIL_CAST_HPP
-#define __UTIL_CAST_HPP
+#pragma once
 
 #include "debug.hpp"
+#include "platform.hpp"
 
 #include <type_traits>
 #include <limits>
@@ -172,9 +172,10 @@ namespace cruft::cast {
     {
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wcast-align"
+#if defined(COMPILER_GCC)
+#pragma GCC diagnostic ignored "-Wcast-function-type"
+#endif
         return reinterpret_cast<DstT> (src);
 #pragma GCC diagnostic pop
     }
 }
-
-#endif