It's now regenerated when util.hh changes, and is ordered after
config.h to fix a race.

Closes #3225.
Closes #3226.

This will eventually be moved to nixos.org.

These are all symlinks to 'nix' now, reducing the installed size by
about ~1.7 MiB.

Note that this only requires headers from boost so it doesn't add a
runtime dependency.

Also, use Nixpkgs 18.03.

In particular, using fetchGit means we don't need hackery to clean the
source tree when building from an unclean tree.

Actually, currently they can only create download activities. Thus,
downloads by builtins.fetchurl show up in the progress bar.

The thread calls poll() to wait until a HUP (or other error event)
happens on the client connection. If so, it sends SIGINT to the main
thread, which is then cleaned up normally. This is much nicer than
messing around with SIGPOLL.

http://hydra.nixos.org/build/9328376