More constexpr and noexcept support.

Note 1: Forwarding functions are specified as a C++14 constexpr since
std::forward is not a constexpr within C++11.

Note 2: Though I'm not sure why it doesn't compile, some declarations
are specified as a C++14 constexpr or non-constexpr.

Note 3: Boost.Tuple adaptation and TR1-based tuple implementations are
not constexpr.

include/boost/fusion/algorithm/transformation/flatten.hpp

@@ -25,13 +25,15 @@ namespace boost { namespace fusion { namespace result_of
 namespace boost { namespace fusion
 {
     template <typename Sequence>
+    BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
     inline typename result_of::flatten<Sequence>::type
     flatten(Sequence& view)
     {
         return flatten_view<Sequence>(view);
     }
-    
+
     template <typename Sequence>
+    BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
     inline typename result_of::flatten<Sequence const>::type
     flatten(Sequence const& view)
     {