none_t is no loner constructible from literal 0

This caused problems because:
optional<T> o = 0;
always worked. But often it would create an uninitialized optional.

include/boost/none_t.hpp

@@ -1,4 +1,5 @@
 // Copyright (C) 2003, Fernando Luis Cacciola Carballal.
+// Copyright (C) 2014 Andrzej Krzemienski.
 //
 // Use, modification, and distribution is subject to the Boost Software
 // License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
@@ -14,9 +15,12 @@
 
 namespace boost {
 
+#ifdef BOOST_OPTIONAL_USE_OLD_DEFINITION_OF_NONE
 namespace detail { struct none_helper{}; }
-
 typedef int detail::none_helper::*none_t ;
+#else
+class none_t {};
+#endif
 
 } // namespace boost