I think what you are looking for is to change the post hook script which will checkout the file called build.xml on every commit that is made to the source code. Or of you do not want to make changes to hook script, the other way might be to add a cron job to checkout build.xml for every hour.
And for build are you using some CI tools? If it is, you can just configure it from your build tool UI to do a build on every commit made on subversion. If you are looking to build using ant may be you should look on something like svnant.