Builds the "zookeeper" ruby gem

This builds the `zookeeper` gem as a package. It also tweaks the `CFLAGS` for the `zookeeper-client` package as it was non-functional as it stood (for me, at least).

I've also added a `buildlink3.mk` for `zookeeper-client` so the build works correctly. I'm new to pkgsrc, so I'm not 100% sure it's correct. But it works for me :)

It's worth pointing out that I've stripped out the internal zk-c build in the gem and replaced with a static-link to the system `libzookeeper_st.a` as there was a linking problem when dynamically linking against it. As I say, works for me.