more robust bootstrap, e.g. when experimenting with Poly/ML repository changes;

--- a/src/Pure/ROOT	Wed Oct 19 18:14:42 2016 +0200
+++ b/src/Pure/ROOT	Thu Oct 20 10:30:11 2016 +0200
@@ -4,6 +4,7 @@
   description {*
     The Pure logical framework
   *}
+  options [threads = 1]
   global_theories
     Pure
   theories