nginx: Allow GD to be optional.

This commit permits incantations like `pkgs.nginx.override { gd = null; }` to
produce a slimmed-down nginx.

When used, this functionality removes a pile of stuff from nginx's closure.
The resulting nginx's closure:

    $ nix-store -q -R /nix/store/wk3h0a4dmdmjmxkbd0q09iw0wfq0yzpz-nginx-1.10.2 | wc -l
    12
    $ nix-store -q -R /nix/store/gpcx77anqrj05qz0mrwm7hf4wgxry5py-nginx-1.10.2 | wc -l
    24