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/clear.hpp

@@ -22,7 +22,7 @@ namespace boost { namespace fusion
     }
 
     template <typename Sequence>
-    BOOST_FUSION_GPU_ENABLED
+    BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
     inline typename result_of::clear<Sequence const>::type
     clear(Sequence const& /*seq*/)
     {