Ok, I think I've figured it out. You create an "empty ant project", right-click, add your files to it, and then double click on the target, and under "Build Tool Configuration" set the Directory to the directory that contains your build.xml file. That should do it (worked for me).
Edit: One more thing: I personally find it easier to not select the checkbox to copy the files over. I just let Xcode create virtual representations of those files, and edit the current ones I have directly.