workaround for occasional deadlock seen in HOL-Proofs with threads=2;

--- a/src/Pure/Isar/locale.ML	Sat Mar 10 14:43:15 2018 +0100
+++ b/src/Pure/Isar/locale.ML	Sat Mar 10 15:52:47 2018 +0100
@@ -420,6 +420,7 @@
 
 fun make_notes kind = map (fn ((b, atts), facts) =>
   if null atts andalso forall (null o #2) facts
+    andalso not (Proofterm.proofs_enabled ()) (* FIXME *)
   then Lazy_Notes (kind, (b, Lazy.value (maps #1 facts)))
   else Notes (kind, [((b, atts), facts)]));