added multithreading, self_critical

--- a/src/Pure/ML-Systems/no_multithreading.ML	Mon Jul 23 22:18:01 2007 +0200
+++ b/src/Pure/ML-Systems/no_multithreading.ML	Mon Jul 23 22:18:03 2007 +0200
@@ -5,6 +5,9 @@
 Compatibility file for ML systems without multithreading.
 *)
 
-(* critical section *)
+(*default number of worker threads*)
+val multithreading = ref (NONE: int option);
 
+(*critical section*)
+fun self_critical () = false;
 fun CRITICAL e = e ();