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/support/iterator_base.hpp

@@ -7,6 +7,7 @@
 #if !defined(FUSION_ITERATOR_BASE_05042005_1008)
 #define FUSION_ITERATOR_BASE_05042005_1008
 
+#include <boost/config.hpp>
 #include <boost/fusion/support/config.hpp>
 
 namespace boost { namespace fusion
@@ -16,16 +17,16 @@ namespace boost { namespace fusion
     template <typename Iterator>
     struct iterator_base : iterator_root
     {
-        BOOST_FUSION_GPU_ENABLED
+        BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
         Iterator const&
-        cast() const
+        cast() const BOOST_NOEXCEPT
         {
             return static_cast<Iterator const&>(*this);
         }
 
-        BOOST_FUSION_GPU_ENABLED
+        BOOST_CXX14_CONSTEXPR BOOST_FUSION_GPU_ENABLED
         Iterator&
-        cast()
+        cast() BOOST_NOEXCEPT
         {
             return static_cast<Iterator&>(*this);
         }