outside of the Nix store for the dependencies of the root source
file, which is impure. That's why it needed the
`builtins.currentTime' hack to force a rebuild. It also didn't work
in a chroot. Now find-includes.pl only scans the source file at
hand, and we use builtins.genericClosure to find all includes
recursively.
svn path=/nixpkgs/trunk/; revision=16414