more robust: allow empty root (e.g. via symlink);

--- a/lib/Tools/ocaml_setup	Tue Mar 19 16:14:59 2019 +0000
+++ b/lib/Tools/ocaml_setup	Wed Mar 20 17:33:06 2019 +0100
@@ -6,7 +6,7 @@
 
 set -e
 
-if [ -d "$ISABELLE_OPAM_ROOT" ]
+if [ -e "$ISABELLE_OPAM_ROOT/config" ]
 then
   isabelle_opam switch "$ISABELLE_OCAML_VERSION"
 else

--- a/lib/scripts/ocamlc	Tue Mar 19 16:14:59 2019 +0000
+++ b/lib/scripts/ocamlc	Wed Mar 20 17:33:06 2019 +0100
@@ -4,7 +4,7 @@
 #
 # Invoke ocamlc via "opam".
 
-if [ -d "$ISABELLE_OPAM_ROOT" ]
+if [ -e "$ISABELLE_OPAM_ROOT/config" ]
 then
   isabelle_opam config exec --switch "$ISABELLE_OCAML_VERSION" -- ocamlc "$@"
 else

--- a/lib/scripts/ocamlexec	Tue Mar 19 16:14:59 2019 +0000
+++ b/lib/scripts/ocamlexec	Wed Mar 20 17:33:06 2019 +0100
@@ -4,7 +4,7 @@
 #
 # Invoke command in OCaml environment setup by "opam".
 
-if [ -d "$ISABELLE_OPAM_ROOT" ]
+if [ -e "$ISABELLE_OPAM_ROOT/config" ]
 then
   isabelle_opam config exec --switch "$ISABELLE_OCAML_VERSION" -- "$@"
 else