[RFC] CMake: Remove shared libraries for messagepack.

The first commit is a quick fix for the "subdirectory problem" pointed out by @jszakmeister in https://github.com/neovim/neovim/issues/1579#issuecomment-64996605. With this, I managed to build an Ubuntu package (although I didn't try to install it yet, so I don't know if it executes).

Also noticed that shared libraries for the new dependencies (libtickit etc) were still there; I tried to adapt the msgpack install script, but it didn't really work out yet.. these changes are in a separate commit.