Fix FTBFS due to g++-4.7
--- a/src/Graphics/xglrgraf.cpp
+++ b/src/Graphics/xglrgraf.cpp
@@ -27,4 +27,5 @@
  */
 #define XGL
 #define FF_GRAPH_SET_PTR
+#include <unistd.h>
 #include "glrgraph.hpp"