Support EasyChair LNCS volume numbers longer than 4 digits

But simply matching `999*` doesn't work, as it would also match the `9999copyrights` directory, so we'd either need to skip that directory in the `for` loops, or use [extended globbing](http://superuser.com/questions/619849/bash-extended-globbing-inside-a-makefile) to match digits only.
