depend on Java 1.6 after all;

--- a/src/Pure/System/isabelle_system.scala	Mon Dec 07 22:23:33 2009 +0100
+++ b/src/Pure/System/isabelle_system.scala	Mon Dec 07 23:06:03 2009 +0100
@@ -260,7 +260,7 @@
   {
     getenv_strict("ISABELLE_TOOLS").split(":").find(dir => {
       val file = platform_file(dir + "/" + name)
-      try { file.isFile && file.canRead } //  file.canExecute requires Java 1.6
+      try { file.isFile && file.canRead && file.canExecute }
       catch { case _: SecurityException => false }
     }) match {
       case Some(dir) =>