Set ant bootclasspath: JDK 1.7 has a new javac warning for setting an older source without bootclasspath

How do I set the ant bootclasspath in conjunction with -source 1.5 -target 1.5?

How can this not be a hardcoded path to the 1.5 JDK? Can I set an environment variable to bootclasspath similar to how JAVA_HOME can be used from ant?

Ideally I would like to do something like set an environment variable or pass an argument to ant.