avoid strict evaluation of "isabelle_stack path --programs";

--- a/lib/scripts/getsettings	Wed Oct 17 21:42:29 2018 +0200
+++ b/lib/scripts/getsettings	Wed Oct 17 21:53:38 2018 +0200
@@ -110,8 +110,10 @@
 fi
 
 #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"
+if [ -d "$ISABELLE_STACK_ROOT" ]; then
+  if [ -f "$(isabelle_stack path --programs)/$ISABELLE_GHC_VERSION/ghc" ]; then
+    ISABELLE_GHC="$ISABELLE_HOME/lib/Tools/ghc"
+  fi
 fi
 
 #enforce JAVA_HOME