Only export statistics(runtime,_) for STATISTICS_RUNTIME

This avoids the need to access the system clock during initialization
of an interpreter, and prevents code from getting the difference
in wall clock time during execution.

src/builtin/PRED_$statistics_2.java

diff --git a/src/builtin/PRED_$statistics_2.java b/src/builtin/PRED_$statistics_2.java
index a0753ac..ab1778f 100644
--- a/src/builtin/PRED_$statistics_2.java
+++ b/src/builtin/PRED_$statistics_2.java
@@ -31,6 +31,7 @@
 	} else if (! a1.isSymbol()) {
 	    throw new IllegalTypeException(this, 1, "atom", a1);
 	} else if (a1.equals(SYM_RUNTIME)) {
+	    engine.requireFeature(Prolog.Feature.STATISTICS_RUNTIME, this, a1);
 	    long val1, val2;
 	    Term start, previous;
 	    val1 = System.currentTimeMillis() - engine.getStartRuntime();

src/lang/Prolog.java

diff --git a/src/lang/Prolog.java b/src/lang/Prolog.java
index 222e37a..1e5d7a2 100644
--- a/src/lang/Prolog.java
+++ b/src/lang/Prolog.java
@@ -156,7 +156,10 @@
       JAVA_REFLECTION,
 
       /** Access to the local filesystem and console. */
-      IO;
+      IO,
+
+      /** Track the running time of evaluations */
+      STATISTICS_RUNTIME;
     }
     protected final EnumSet<Feature> features = EnumSet.allOf(Feature.class);
 
@@ -251,8 +254,10 @@
 	doubleQuotes    = "codes";
 	printStackTrace = "off";
 
-	exception       = NONE;
-	startRuntime    = System.currentTimeMillis();
+	exception = NONE;
+	startRuntime = features.contains(Feature.STATISTICS_RUNTIME)
+	    ? System.currentTimeMillis()
+	    : 0;
 	previousRuntime = 0;
 
 	userOutput.flush();