ocaml 4.12 compat

Index: src/networks/donkey/donkeyClient.ml
--- src/networks/donkey/donkeyClient.ml.orig
+++ src/networks/donkey/donkeyClient.ml
@@ -818,8 +818,7 @@ another better way, since this functionnality is still
         end
 
 
-external hash_param : int -> int -> 'a -> int = "caml_hash_univ_param" "noalloc"
-let hash x = hash_param 10 100 x
+let hash = Hashtbl.hash
 
 let shared_of_file file =
   match file.file_shared with
