Initial constexpr support

This allows to use, e.g., boost::fusion::fold within constexpr functions.

The BOOST_CONSTEXPR macro is used to declare functions constexpr.

include/boost/fusion/iterator/next.hpp

@@ -54,7 +54,7 @@ namespace boost { namespace fusion
     }
 
     template <typename Iterator>
-    BOOST_FUSION_GPU_ENABLED
+    BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
     typename result_of::next<Iterator>::type const
     next(Iterator const& i)
     {