Stops the ant-contrib library being downloaded to invalid locations

* On occasion the ant-contrib library can get downloaded to
  ${ant.library.dir} when this property has no value. Avoid this by
  just checking on the value of this property prior to downloading

scripts/build-common.xml

@@ -18,7 +18,7 @@
          of the eclipse installation,
          eg. /opt/eclipse/plugins/org.apache.ant_1.8.4.v201303080030/lib/
       -->
-    <target name="download-ant-contrib" unless="ant-contrib-exists">
+    <target name="download-ant-contrib" if="ant.library.dir" unless="ant-contrib-exists">
         <echo message="Downloading ant-contrib library"/>
 
         <!-- Create the ant lib directory if not already exists -->