multithreading.ML provides dummy thread structures;

--- a/src/Pure/ML-Systems/polyml.ML	Thu Sep 04 16:03:47 2008 +0200
+++ b/src/Pure/ML-Systems/polyml.ML	Thu Sep 04 16:03:48 2008 +0200
@@ -4,7 +4,10 @@
 Compatibility wrapper for Poly/ML (after 5.1).
 *)
 
+structure PolyML_Thread = Thread;
 use "ML-Systems/polyml_common.ML";
+
+open PolyML_Thread;
 use "ML-Systems/multithreading_polyml.ML";
 
 val pointer_eq = PolyML.pointerEq;