Fix compilation problem with older GCC versions

src/graph/generation/graph_geometric.hh

@@ -19,7 +19,12 @@
 #define GRAPH_GEOMETRIC_HH
 
 #include <iostream>
-#include <tr1/unordered_map>
+#if (GCC_VERSION >= 40400)
+#   include <tr1/unordered_map>
+#else
+#   include <boost/tr1/unordered_map.hpp>
+#endif
+
 #include <boost/functional/hash.hpp>
 #include "graph_util.hh"