Ant's symlink task seems to have trouble if the link already exists. So any attempt to build for a second time fails. We use symlink to link version specific JavaCC jar to javacc.jar. Curiously, even if you succeed in symlink for javacc, javacc task fails with an error that javacc.jar or JavaCC.zip cannot be found in path. It appears that the ant problem has been around for some time: http://firstname.lastname@example.org/msg00880.html.
The forthcoming patch addresses the problem by replacing symlink with a delete and copy. I could have simply used copy with overwrite="true", but, I'm not sure how it'll behave with existing symlinks.