Well, I guess I'm going to press on with trying to build Jython for compatibility with earlier versions of Java. I'm on linux ( ubuntu ). I'm completely unfamiliar with these particular tools. So far,

I've downloaded the latest Jython source posted on http://www.jython.org/downloads.html

I've installed "ant" and "antlr3" using apt-get.
At first, I got a NoClassDefFoundError for org.antlr.Tool, which was solved by export CLASSPATH="$CLASSPATH:/usr/share/java/*"

So now, I can call
$ ant developer-build

and it at least tries to build, but I get a lot of errors like

package javax.servlet does not exist
package org.antlr.runtime does not exist
cannot find symbol
    [javac] symbol  : class ParserRuleReturnScope
...

So presumably $CLASSPATH is set up well enough for ANT to run, but not correctly, since these various libraries aren't found during compilation?
I'll keep working on this but it will probably take me a bit of time to learn all of the tools.