Add 'javatarget' attribute to ant task so that it behaves like ant javac task

I am on a machine running the 1.5 JDK. However, I must generate xmlbean jar files that are compatible with 1.4 JDKs. I expected there to be an attribute similar to that found in the ant javac task that allows one to generate class files for specific VM version, like so:

<javac srcdir="${src}"
destdir="${build}"
source="1.4"
target="1.4"
/>

I thought that setting the javasource attribute might have this effect, but it does not. As a result of not having this feature, I am unable to generate compatible jars from within ant using my current build process which makes use of the above-mentioned javac attribute.