Systems such as Debian that have overridden the default fpath will need to
set ZSH_COMPLETIONS_PATH.

I feel that Debian is causing unncessary complexity by making this change,
and have filed a bug report about it.

This also means that when git-annex is installed with PREFIX=/usr/local
it will use /usr/local/share/zsh/site-functions which works with probably
all versions of zsh.