build: Install zsh completion

[danielshahaf]

Fixes #534

[jay]

did you copy this from nghttp2? this line needs to be changed, also there is another line farther below. did you test this?

[danielshahaf]

I tested the --with-with-argument and --with-without-argument codepaths but not the --without- codepath. Fixed.

[bagder]

Is zsh used commonly enough for this to be done by default?

[danielshahaf]

I don't know where to find hard data on zsh's popularity. (There's the debian popcon data and systemd's default behaviour with respect to their zsh completion functions, but I don't know whether either of these datapoints is representative of anything.)

I suppose that, if --with-zsh-functions-dir isn't passed, you have three options:

• Unconditionally install the completion.
• Unconditionally do not install the completion.
• Check whether /usr/local/share/zsh exists and only install the completion if it does.
  This may lead to unpredictable behaviour if zsh is installed sometimes before curl and sometimes after it.

[bagder]

Thanks! I merged your version now so it will install it unconditionally. Let's see if we get any protests...

[danielshahaf]

Thanks @bagder!