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/sequence/comparison/less.hpp

@@ -18,7 +18,7 @@
 namespace boost { namespace fusion
 {
     template <typename Seq1, typename Seq2>
-    BOOST_FUSION_GPU_ENABLED
+    BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
     inline bool
     less(Seq1 const& a, Seq2 const& b)
     {
@@ -29,7 +29,7 @@ namespace boost { namespace fusion
     namespace operators
     {
         template <typename Seq1, typename Seq2>
-        BOOST_FUSION_GPU_ENABLED
+        BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
         inline typename
             boost::enable_if<
                 traits::enable_comparison<Seq1, Seq2>