+# define _GNU_SOURCE
+#endif
+
+// Teach the compiler that some code path is unreacheable:
+#if defined(__has_builtin)
+# if __has_builtin(__builtin_unreachable)
+# define XBT_UNREACHABLE() __builtin_unreachable()
+# else
+# include <stdlib.h>
+# define XBT_UNREACHABLE() abort()
+# endif
+#elif (__GNUC__ > 4) || (__GNUC__ == 4 && __GNUC_MINOR__ >= 5)
+# define XBT_UNREACHABLE() __builtin_unreachable()
+#else
+# include <stdlib.h>
+# define XBT_UNREACHABLE() abort()