Fix: create the dir when -dir is used

lib/lv6MainArgs.ml

-(* Time-stamp: <modified the 05/07/2021 (at 10:34) by Erwan Jahier> *)
+(* Time-stamp: <modified the 05/07/2021 (at 16:40) 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,
@@ -282,7 +282,9 @@ let mkoptab (opt:t) : unit = (
       ["Set the output file name"]
     ;
     mkopt opt  ["-dir";"--directory"]  ~arg:" <string>"
-      (Arg.String(function x -> global_opt.dir <- x))
+      (Arg.String(function x ->
+                    Sys.mkdir x 0o640;
+                    global_opt.dir <- x))
       ["Set the directory into which output files are generated"]
     ;
     mkopt opt