I was looking at the build.xml and have 2 comments.
a) Since SQuirreL will not run on a JVM <1.3 why not set the target attribute in the javac task in ant to 1.3 (as in target="1.3") ? this will abort any attempt to run it on a jvm < 1.3. This will prevent bug reports for lower JVMs. It might also be a good idea to set the bootclasspath to the 1.4 rt.jar (see link below)
b) includeJavaRuntime="yes" is not needed (nor is it recommended, see link below) . All this does is add rt.jar to the classpath.
For more on the subject I recommend reading: