Make Makefile more configurable (doc and zsh dirs)

It's only a matter of time until vcsh lands in Fedora [1] but packaging it needed some changes in the Makefile. I have kept it to the bare minimum, that is to say a `docdir` and a `zshdir` even though standard macros such as `bindir` or `mandir` exist (but are less likely to vary between various distributions.

[1] https://bugzilla.redhat.com/show_bug.cgi?id=vcsh