unset ISABELLE_SETTINGS_PRESENT;

--- a/lib/Tools/usedir	Thu Oct 07 15:40:32 1999 +0200
+++ b/lib/Tools/usedir	Thu Oct 07 17:19:07 1999 +0200
@@ -54,6 +54,7 @@
     case "$OPT" in
       B)
         BUILD=true
+        unset ISABELLE_SETTINGS_PRESENT
         export THIS_IS_ISABELLE_BUILD=true
         ;;
       b)