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_equal.hpp

@@ -24,7 +24,7 @@
 namespace boost { namespace fusion
 {
     template <typename Seq1, typename Seq2>
-    BOOST_FUSION_GPU_ENABLED
+    BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
     inline bool
     less_equal(Seq1 const& a, Seq2 const& b)
     {
@@ -41,7 +41,7 @@ namespace boost { namespace fusion
 #if defined(BOOST_MSVC) && (BOOST_MSVC <= 1400)
 // Workaround for  VC8.0 and VC7.1
         template <typename Seq1, typename Seq2>
-        BOOST_FUSION_GPU_ENABLED
+        BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
         inline bool
         operator<=(sequence_base<Seq1> const& a, sequence_base<Seq2> const& b)
         {
@@ -49,7 +49,7 @@ namespace boost { namespace fusion
         }
 
         template <typename Seq1, typename Seq2>
-        BOOST_FUSION_GPU_ENABLED
+        BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
         inline typename disable_if<traits::is_native_fusion_sequence<Seq2>, bool>::type
         operator<=(sequence_base<Seq1> const& a, Seq2 const& b)
         {
@@ -57,7 +57,7 @@ namespace boost { namespace fusion
         }
 
         template <typename Seq1, typename Seq2>
-        BOOST_FUSION_GPU_ENABLED
+        BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
         inline typename disable_if<traits::is_native_fusion_sequence<Seq1>, bool>::type
         operator<=(Seq1 const& a, sequence_base<Seq2> const& b)
         {
@@ -69,7 +69,7 @@ namespace boost { namespace fusion
 // but barfs somewhere else.
 
         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>