I built xfce4-4.10 (up to thunar on that same page you linked) in the past, also on -current before it was added to -current. It worked fine.
If you read the list of what to build, you'll see two things:
1. pkgconfig is only used by xfce4-dev-tools, which isn't required for the other packages.
2. coffee is not a dependency for any of those packages
I'm not sure what each one does either, but I recall I built it without reading the "Build requirements" section because Slackware had Xfce and so I assumed all the "Build requirements" were already met, hehe.