dev-java/javacup: Use installed javacup over bundled where possible

dev-java/javacup: Use installed javacup over bundled where possible

Fixes bug #565338.

Annoyingly javacup bundles an older version of itself that will break
jflex if that has already been built against this newer version
beforehand. Even more annoyingly, the binary download is built with
Java 8, which isn't much use to us. We therefore use any installed
javacup that is newer than the bundled version where possible. If this
approach turns out to be unworkable then we may just have to use the
bundled jflex for bootstrapping.

Package-Manager: portage-2.2.20.1