specifying dataDir on launch of jetty

[Brian Whitman]

I would like to specify the solr dataDir on launch of jetty via java - 
jar start.jar instead of editing the solrconfig.xml before launching.

I've tried java -Dsolr.dataDir=/x/y/z -jar start.jar but it seems to  
have no effect -- it starts with the solrconfig.xml default.

Use case is that I would like to deploy solr to other users with the  
data dir going in their home directories.. setting the dataDir in  
solrconfig.xml to ~/solrdata does not work (the ~ does not get  
expanded on this OSX system.)