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/view/nview/nview_iterator.hpp

@@ -42,7 +42,8 @@ namespace boost { namespace fusion
         typedef Sequence sequence_type;
         typedef mpl_iterator<Pos> first_type;
 
-        BOOST_FUSION_GPU_ENABLED explicit nview_iterator(Sequence& in_seq)
+        BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
+        explicit nview_iterator(Sequence& in_seq)
           : seq(in_seq) {}
 
         Sequence& seq;