builder: Old OCaml didn't have List.iteri, so add a utility function.

builder/builder.ml

@@ -50,7 +50,7 @@ let main () =
     eprintf "command line:";
     List.iter (eprintf " %s") (Array.to_list Sys.argv);
     prerr_newline ();
-    List.iteri (
+    iteri (
       fun i (source, fingerprint) ->
         eprintf "source[%d] = (%S, %S)\n" i source fingerprint
     ) sources

mllib/common_utils.ml

@@ -164,6 +164,13 @@ let rec filter_map f = function
       | Some y -> y :: filter_map f xs
       | None -> filter_map f xs
 
+let iteri f xs =
+  let rec loop i = function
+    | [] -> ()
+    | x :: xs -> f i x; loop (i+1) xs
+  in
+  loop 0 xs
+
 (* Timestamped progress messages, used for ordinary messages when not
  * --quiet.
  *)