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/container/set/detail/convert_impl.hpp

@@ -34,7 +34,7 @@ namespace boost { namespace fusion
                     template apply<typename result_of::begin<Sequence>::type>::type
                 type;
 
-                BOOST_FUSION_GPU_ENABLED
+                BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
                 static type call(Sequence& seq)
                 {
                     return gen::call(fusion::begin(seq));