fop should depend on ant-optional too, it now emerges directly after ant and thus complains about TraXLiaison (part of ant-optional) not being installed. The only thing which should definately be looked at... the dependencies of ant-optional incompare with fop.
I didn't see this on my system, but I will investigater close. In the meantime, can you supply us with a complete error dump?
Sorry I didn't save the logs. But the main this was it couldn't compile because of the missing TraXLiaison class. Pretty much because it wasn't installed :) So this isn't a Java thing just an ebuild what needs some work.
this should be fixed with the ant/ant-core splitup