adding quickcheck's potential configuration

--- a/src/Tools/quickcheck.ML	Tue Nov 29 22:45:21 2011 +0100
+++ b/src/Tools/quickcheck.ML	Wed Nov 30 09:21:02 2011 +0100
@@ -20,6 +20,7 @@
   val no_assms : bool Config.T
   val report : bool Config.T
   val timing : bool Config.T
+  val potential : bool Config.T
   val quiet : bool Config.T
   val timeout : real Config.T
   val allow_function_inversion : bool Config.T;
@@ -170,6 +171,7 @@
 val no_assms = Attrib.setup_config_bool @{binding quickcheck_no_assms} (K false)
 val report = Attrib.setup_config_bool @{binding quickcheck_report} (K true)
 val timing = Attrib.setup_config_bool @{binding quickcheck_timing} (K false)
+val potential = Attrib.setup_config_bool @{binding quickcheck_potential} (K true)
 val quiet = Attrib.setup_config_bool @{binding quickcheck_quiet} (K false)
 val timeout = Attrib.setup_config_real @{binding quickcheck_timeout} (K 30.0)
 val allow_function_inversion =