-# -DUSG If you have System V/ANSI C string
-# and memory functions and headers,
-# fcntl.h, getcwd, no valloc,
-# and ndir.h (unless you use -DDIRENT).
-# -DDIRENT If USG and you have dirent.h instead of ndir.h.
+# -DDIRENT If you have dirent.h.
+# -DSYSNDIR Old Xenix systems (sys/ndir.h).
+# -DSYSDIR Old BSD systems (sys/dir.h).
+# -DNDIR Old System V systems (ndir.h).