MaxOS fixes

Avoid a thread deadlock
improvements to SWI predicates.
make variables_in_term system builtin.


git-svn-id: https://yap.svn.sf.net/svnroot/yap/trunk@2304 b08c6af1-5177-4d33-ba66-4b1c6b8b522a

C/threads.c

@@ -897,6 +897,11 @@ p_thread_self(void)
   return Yap_unify(ARG1,MkIntTerm(0));
 }
 
+p_thread_stacks(void)
+{				/* '$thread_runtime'(+P)	 */
+  return FALSE;
+}
+
 static Int 
 p_thread_unlock(void)
 {				/* '$thread_runtime'(+P)	 */