revoman,
I think the modern compiler is new with Java 6. What you need to do is edit your Ant runtime configuration, go to the JRE tab, and set the JRE to run your Ant script with to some Java 6 install you have on your local machine.
Then you should be all set.
This will likely “automatically work” in MyEclipse 7.5 and later for you because we changed the default JRE to run MyEclipse to 1.6.0_13