title: How to execute an Ant task only when source files have been modified?
body: <p>There must be an easy way to do this.  I build a Flex app using ant that depends on a SWC library, which works fine except that it rebuilds the library whether it needs to or not.  How do I tell ant to only run the task if any of the sources files of the library (*.as, *.mxml) are newer than the SWC?  </p>
  
  <p>I've looked at &lt;dependset> but it only seems to delete files, not determine whether a task should be run or not.  &lt;depend> seems to expect a one-to-one relationship between the source and target files rather than a one-to-many relationship -- I have many input files and one output file, but no intermediate object files.</p>
  
  <p>Thanks a lot,
  Alex</p>