only test prolog code examples if environment variable is set

--- a/src/HOL/Predicate_Compile_Examples/ROOT.ML	Mon Aug 02 22:24:19 2010 +0200
+++ b/src/HOL/Predicate_Compile_Examples/ROOT.ML	Tue Aug 03 08:23:08 2010 +0200
@@ -1,2 +1,2 @@
 use_thys ["Predicate_Compile_Examples", "Predicate_Compile_Quickcheck_Examples", "Specialisation_Examples"];
-use_thys ["Code_Prolog_Examples"];
+if getenv "PROLOG_HOME" = "" then () else use_thys ["Code_Prolog_Examples"];