Only try to use clover if there's a license

Fixes CORE-7

Change-Id: Ib25a6ae0664a3bccc2fe1d4f598d557e72f4c9d7
Reviewed-on: http://gerrit.dmdirc.com/1519
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: Chris Smith <chris@dmdirc.com>

build-tests.xml

@@ -5,9 +5,15 @@
     <property name="clover.jar" location="lib/clover.jar"/>
     <taskdef resource="cloverlib.xml" classpath="${clover.jar}"/>
     <taskdef resource="cloverjunitlib.xml" classpath="${clover.jar}"/>
-    <available property="clover.installed" classname="com.cenqua.clover.CloverInstr" />
     <available file="/usr/bin/xvfb-run" property="has.xvfb"/>
 
+    <condition property="clover.installed">
+     <and>
+      <available classname="com.cenqua.clover.CloverInstr"/>
+      <available file="lib/clover.license"/>
+     </and>
+    </condition>
+
     <target name="with.clover" depends="clean" if="clover.installed"
             description="Set up Clover to instrument tests.">
         <clover-clean/>