My build script executes:
Assuming that you're running Jenkins on your iMac, go to [http://yourjenkins/configure] and add an Ant installation that points to your Ant 1.8.4 directory. Then, in the Ant build step of your Jenkins job configuration, specify that version of Ant and things should start working.
If your Jenkins server is on a different machine, repeat the instructions but install Ant 1.8.4 on the Jenkins server, or use the "Install automatically" option to install Ant on demand.