If your corporate firewall is blocking access to the source code repository, it is probably also blocking access to the third party library (Maven) repository. By the way, the source is now in Subversion at http://anonsvn.jboss.org/repos/portal/.
Could you try doing this from home (or somewhere outside the corporate firewall) and see if it works there.
You could also enter http://repository.jboss.com/antlr/2.7.6.ga/ into your browser to see if you get the directory listing, and then click on the component-info.xml file and see if that gets displayed. If you cannot do that, then you will *not* be able to download that same file from the build.