I think this may have come from a branch merge. In this commit:
A line was added which unzips the dist zip file to build/ in the toplevel directory. However if the build/ directory is already populated with the contents of the dist zip file (through ant dist), unzip will prompt to overwrite the files, causing the build process to hang as it is unable to respond. This patch addresses this by forcing the overwrite of files when running the unzip task.