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/generation/ignore.hpp

@@ -17,7 +17,7 @@ namespace boost { namespace fusion
         struct swallow_assign
         {
             template<typename T>
-            BOOST_FUSION_GPU_ENABLED
+            BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
             swallow_assign const&
             operator=(const T&) const
             {
@@ -27,7 +27,7 @@ namespace boost { namespace fusion
     }
 
     //  "ignore" allows tuple positions to be ignored when using "tie".
-    detail::swallow_assign const ignore = detail::swallow_assign();
+    BOOST_CONSTEXPR detail::swallow_assign const ignore = detail::swallow_assign();
 }}
 
 #endif