enforce settings that are likely to be outdated, e.g. in $ISABELLE_HOME_USER/etc/settings;

--- a/lib/scripts/getsettings	Wed Oct 17 21:38:07 2018 +0200
+++ b/lib/scripts/getsettings	Wed Oct 17 21:42:29 2018 +0200
@@ -99,16 +99,18 @@
   ML_IDENTIFIER="${ML_SYSTEM}_${ML_PLATFORM}"
 fi
 
-#OCaml
-if [ -z "$ISABELLE_OCAML" -a -f "$ISABELLE_OPAM_ROOT/$ISABELLE_OCAML_VERSION/bin/ocaml" ]; then
+#enforce ISABELLE_OCAML
+if [ -f "$ISABELLE_OPAM_ROOT/$ISABELLE_OCAML_VERSION/bin/ocaml" ]; then
   ISABELLE_OCAML="$ISABELLE_HOME/lib/Tools/ocaml"
 fi
-if [ -z "$ISABELLE_OCAMLC" -a -f "$ISABELLE_OPAM_ROOT/$ISABELLE_OCAML_VERSION/bin/ocamlc" ]; then
+
+#enforce ISABELLE_OCAMLC
+if [ -f "$ISABELLE_OPAM_ROOT/$ISABELLE_OCAML_VERSION/bin/ocamlc" ]; then
   ISABELLE_OCAMLC="$ISABELLE_HOME/lib/Tools/ocamlc"
 fi
 
-#GHC
-if [ -z "$ISABELLE_GHC" -a -d "$ISABELLE_STACK_ROOT" -a -f "$(isabelle_stack path --programs)/$ISABELLE_GHC_VERSION/ghc" ]; then
+#enforce ISABELLE_GHC
+if [ -d "$ISABELLE_STACK_ROOT" -a -f "$(isabelle_stack path --programs)/$ISABELLE_GHC_VERSION/ghc" ]; then
   ISABELLE_GHC="$ISABELLE_HOME/lib/Tools/ghc"
 fi