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/query/count.hpp

@@ -26,9 +26,8 @@ namespace boost { namespace fusion
     }
 
     template <typename Sequence, typename T>
-    BOOST_FUSION_GPU_ENABLED
-    inline
-    typename
+    BOOST_CXX14_CONSTEXPR BOOST_FUSION_GPU_ENABLED
+    inline typename
         enable_if<
             traits::is_sequence<Sequence>
           , int