build faster without heap images for minor imports;

--- a/src/HOL/ROOT	Wed Nov 01 18:24:39 2017 +0100
+++ b/src/HOL/ROOT	Wed Nov 01 18:37:49 2017 +0100
@@ -927,13 +927,15 @@
     "Examples/Finite"
     "Examples/T2_Spaces"
 
-session HOLCF (main timing) in HOLCF = "HOL-Library" +
+session HOLCF (main timing) in HOLCF = HOL +
   description {*
     Author:     Franz Regensburger
     Author:     Brian Huffman
 
     HOLCF -- a semantic extension of HOL by the LCF logic.
   *}
+  sessions
+    "HOL-Library"
   theories
     HOLCF (global)
   document_files "root.tex"