environment variable check has become pointless after 771b8ad5c7fc

--- a/src/Tools/Code/code_ml.ML	Tue Apr 19 13:05:50 2016 +0200
+++ b/src/Tools/Code/code_ml.ML	Mon Apr 18 20:56:11 2016 +0200
@@ -868,7 +868,7 @@
 val _ = Theory.setup
   (Code_Target.add_language
     (target_SML, { serializer = serializer_sml, literals = literals_sml,
-      check = { env_var = "ISABELLE_TOOL",
+      check = { env_var = "",
         make_destination = fn p => Path.append p (Path.explode "ROOT.ML"),
         make_command = fn _ =>
           "isabelle process -e 'datatype ref = datatype Unsynchronized.ref' -f 'ROOT.ML' -l Pure" } })

--- a/src/Tools/Code/code_target.ML	Tue Apr 19 13:05:50 2016 +0200
+++ b/src/Tools/Code/code_target.ML	Mon Apr 18 20:56:11 2016 +0200
@@ -389,7 +389,7 @@
         else ()
       end;
   in
-    if getenv env_var = ""
+    if not (env_var = "") andalso getenv env_var = ""
     then if strict
       then error (env_var ^ " not set; cannot check code for " ^ target_name)
       else warning (env_var ^ " not set; skipped checking code for " ^ target_name)