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/as_set.hpp

@@ -37,7 +37,7 @@ BOOST_FUSION_BARRIER_BEGIN
         };
 
         template <typename Iterator>
-        BOOST_FUSION_GPU_ENABLED
+        BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
         static typename apply<Iterator>::type
         call(Iterator)
         {
@@ -123,7 +123,7 @@ BOOST_FUSION_BARRIER_END
         };
 
         template <typename Iterator>
-        BOOST_FUSION_GPU_ENABLED
+        BOOST_CXX14_CONSTEXPR BOOST_FUSION_GPU_ENABLED
         static typename apply<Iterator>::type
         call(Iterator const& i0)
         {

include/boost/fusion/container/set/detail/begin_impl.hpp

@@ -32,7 +32,7 @@ namespace boost { namespace fusion { namespace extension
                 >
             type;
 
-            BOOST_FUSION_GPU_ENABLED
+            BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
             static type
             call(Seq& seq)
             {

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));

include/boost/fusion/container/set/detail/deref_impl.hpp

@@ -34,7 +34,7 @@ namespace boost { namespace fusion { namespace extension
                 >::type
             type;
 
-            BOOST_FUSION_GPU_ENABLED
+            BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
             static type
             call(It const& it)
             {

include/boost/fusion/container/set/detail/end_impl.hpp

@@ -32,7 +32,7 @@ namespace boost { namespace fusion { namespace extension
                 >
             type;
 
-            BOOST_FUSION_GPU_ENABLED
+            BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
             static type
             call(Seq& seq)
             {

include/boost/fusion/container/set/detail/set_forward_ctor.hpp

@@ -27,7 +27,7 @@
 
 #define N BOOST_PP_ITERATION()
 
-    BOOST_FUSION_GPU_ENABLED
+    BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
 #if N == 1
     explicit
 #endif