Make boost::coroutine optional

This is essentially because of a Debian bug, where no boost::coroutine
shared libraries are available.

configure.ac

@@ -217,7 +217,7 @@ if test "$BOOST_REGEX_LIB" = ""; then
 fi
 AX_BOOST_COROUTINE
 if test "$BOOST_COROUTINE_LIB" = ""; then
-   AC_MSG_ERROR([No usable boost::coroutine found])
+   AC_MSG_WARN([No usable boost::coroutine found! Some functionality will be disabled in the library.])
 fi
 AX_BOOST_GRAPH
 if test "$BOOST_GRAPH_LIB" = ""; then

src/graph/search/graph_astar.cc

@@ -153,6 +153,7 @@ boost::python::object astar_search_generator(GraphInterface& g,
                                              python::object inf,
                                              python::object h)
 {
+#ifdef HAVE_BOOST_COROUTINE
     auto dispatch = [&](auto& yield)
         {
             AStarGeneratorVisitor vis(g, yield);
@@ -164,6 +165,9 @@ boost::python::object astar_search_generator(GraphInterface& g,
                 writable_vertex_properties())(dist_map);
         };
     return boost::python::object(AStarGenerator(dispatch));
+#else
+    throw GraphException("This functionality is not available because boost::coroutine was not found at compile-time")
+#endif
 }
 
 boost::python::object astar_search_generator_fast(GraphInterface& g,

src/graph/search/graph_bfs.cc

@@ -164,6 +164,7 @@ private:
 
 boost::python::object bfs_search_generator(GraphInterface& g, size_t s)
 {
+#ifdef HAVE_BOOST_COROUTINE
     auto dispatch = [&](auto& yield)
         {
             BFSGeneratorVisitor vis(g, yield);
@@ -171,6 +172,9 @@ boost::python::object bfs_search_generator(GraphInterface& g, size_t s)
                 (g, std::bind(do_bfs(), placeholders::_1, s, vis))();
         };
     return boost::python::object(BFSGenerator(dispatch));
+#else
+    throw GraphException("This functionality is not available because boost::coroutine was not found at compile-time")
+#endif
 }
 
 void export_bfs()

src/graph/search/graph_dfs.cc

@@ -154,6 +154,7 @@ private:
 
 boost::python::object dfs_search_generator(GraphInterface& g, size_t s)
 {
+#ifdef HAVE_BOOST_COROUTINE
     auto dispatch = [&](auto& yield)
         {
             DFSGeneratorVisitor vis(g, yield);
@@ -162,6 +163,9 @@ boost::python::object dfs_search_generator(GraphInterface& g, size_t s)
                               g.get_vertex_index(), s, vis))();
         };
     return boost::python::object(DFSGenerator(dispatch));
+#else
+    throw GraphException("This functionality is not available because boost::coroutine was not found at compile-time")
+#endif
 }
 
 void export_dfs()

src/graph/search/graph_dijkstra.cc

@@ -231,6 +231,7 @@ boost::python::object dijkstra_search_generator(GraphInterface& g,
                                                 python::object zero,
                                                 python::object inf)
 {
+#ifdef HAVE_BOOST_COROUTINE
     auto dispatch = [&](auto& yield)
         {
             DJKGeneratorVisitor vis(g, yield);
@@ -242,6 +243,9 @@ boost::python::object dijkstra_search_generator(GraphInterface& g,
              writable_vertex_properties())(dist_map);
         };
     return boost::python::object(DJKGenerator(dispatch));
+#else
+    throw GraphException("This functionality is not available because boost::coroutine was not found at compile-time")
+#endif
 }
 
 boost::python::object dijkstra_search_generator_fast(GraphInterface& g,
@@ -250,6 +254,7 @@ boost::python::object dijkstra_search_generator_fast(GraphInterface& g,
                                                      boost::any weight,
                                                      python::object zero, python::object inf)
 {
+#ifdef HAVE_BOOST_COROUTINE
     auto dispatch = [&](auto& yield)
         {
             DJKGeneratorVisitor vis(g, yield);
@@ -261,6 +266,9 @@ boost::python::object dijkstra_search_generator_fast(GraphInterface& g,
              edge_scalar_properties())(dist_map, weight);
         };
     return boost::python::object(DJKGenerator(dispatch));
+#else
+    throw GraphException("This functionality is not available because boost::coroutine was not found at compile-time")
+#endif
 }
 
 void export_dijkstra()