Use "compopt -o filenames" if available instead of "compgen -f" hack in _cd().

bash_completion

@@ -1370,8 +1370,10 @@ _cd()
         return 0
     fi
 
-    # Enable -o filenames option, see Debian bug #272660
-    compgen -f /non-existing-dir/ >/dev/null
+    # Turn on -o filenames; see http://bugs.debian.org/272660#64 for
+    # info about the compgen hack (bash < 4)
+    compopt -o filenames 2>/dev/null || \
+        compgen -f /non-existing-dir/ >/dev/null
 
     # Use standard dir completion if no CDPATH or parameter starts with /,
     # ./ or ../