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/adapted/std_tuple/detail/at_impl.hpp

@@ -40,7 +40,7 @@ namespace boost { namespace fusion
                     >::type
                 type;
 
-                BOOST_FUSION_GPU_ENABLED
+                BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
                 static type
                 call(Sequence& seq)
                 {

include/boost/fusion/adapted/std_tuple/detail/begin_impl.hpp

@@ -27,7 +27,7 @@ namespace boost { namespace fusion
             {
                 typedef std_tuple_iterator<Sequence, 0> type;
 
-                BOOST_FUSION_GPU_ENABLED
+                BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
                 static type
                 call(Sequence& v)
                 {

include/boost/fusion/adapted/std_tuple/detail/build_std_tuple.hpp

@@ -27,7 +27,7 @@ namespace boost { namespace fusion { namespace detail
     struct build_std_tuple<First, Last, true>
     {
         typedef std::tuple<> type;
-        BOOST_FUSION_GPU_ENABLED
+        BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
         static type
         call(First const&, Last const&)
         {
@@ -61,14 +61,14 @@ namespace boost { namespace fusion { namespace detail
         typedef std::tuple<T, Rest...> type;
 
         template <int ...I>
-        BOOST_FUSION_GPU_ENABLED
+        BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
         static type
         indexed_call(T const& first, std::tuple<Rest...> const& rest, indexed_tuple<I...>)
         {
             return type(first, std::get<I>(rest)...);
         }
 
-        BOOST_FUSION_GPU_ENABLED
+        BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
         static type
         call(T const& first, std::tuple<Rest...> const& rest)
         {
@@ -91,7 +91,7 @@ namespace boost { namespace fusion { namespace detail
 
         typedef typename push_front::type type;
 
-        BOOST_FUSION_GPU_ENABLED
+        BOOST_CXX14_CONSTEXPR BOOST_FUSION_GPU_ENABLED
         static type
         call(First const& f, Last const& l)
         {

include/boost/fusion/adapted/std_tuple/detail/convert_impl.hpp

@@ -34,7 +34,7 @@ namespace boost { namespace fusion
 
                 typedef typename gen::type type;
 
-                BOOST_FUSION_GPU_ENABLED
+                BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
                 static type
                 call(Sequence& seq)
                 {

include/boost/fusion/adapted/std_tuple/detail/end_impl.hpp

@@ -31,7 +31,7 @@ namespace boost { namespace fusion
                 static int const size = std::tuple_size<seq_type>::value;
                 typedef std_tuple_iterator<Sequence, size> type;
 
-                BOOST_FUSION_GPU_ENABLED
+                BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
                 static type
                 call(Sequence& v)
                 {