multithreading support only for polyml-5.2.1 or later;

--- a/NEWS	Thu Oct 23 15:28:05 2008 +0200
+++ b/NEWS	Thu Oct 23 15:28:08 2008 +0200
@@ -336,8 +336,8 @@
 
 *** System ***
 
-* Multithreading for Poly/ML 5.1 is no longer supported, only for
-Poly/ML 5.2 or later.
+* Multithreading for Poly/ML 5.1/5.2 is no longer supported, only for
+Poly/ML 5.2.1 or later.
 
 * The Isabelle "emacs" tool provides a specific interface to invoke
 Proof General / Emacs, with more explicit failure if that is not

--- a/src/Pure/ML-Systems/polyml.ML	Thu Oct 23 15:28:05 2008 +0200
+++ b/src/Pure/ML-Systems/polyml.ML	Thu Oct 23 15:28:08 2008 +0200
@@ -6,7 +6,9 @@
 
 open Thread;
 use "ML-Systems/polyml_common.ML";
-use "ML-Systems/multithreading_polyml.ML";
+
+if ml_system = "polyml-5.2" then ()
+else use "ML-Systems/multithreading_polyml.ML";
 
 val pointer_eq = PolyML.pointerEq;