Fix: create the dir with -dir only if it does not exist

lib/lv6MainArgs.ml

-(* Time-stamp: <modified the 05/07/2021 (at 16:40) by Erwan Jahier> *)
+(* Time-stamp: <modified the 05/07/2021 (at 16:48) by Erwan Jahier> *)
 (*
 Le manager d'argument adapt de celui de lutin, plus joli
 N.B. solution un peu batarde : les options sont stockes, comme avant, dans Global,
@@ -283,7 +283,7 @@ let mkoptab (opt:t) : unit = (
     ;
     mkopt opt  ["-dir";"--directory"]  ~arg:" <string>"
       (Arg.String(function x ->
-                    Sys.mkdir x 0o640;
+                    if not (Sys.file_exists x) then Sys.mkdir x 0o744;
                     global_opt.dir <- x))
       ["Set the directory into which output files are generated"]
     ;