* Run `test' on `make check'.

  won't see header files under Nix control.

* Cleaned up command-line syntax.

  entries by name.

* Lots of refactorings.
* Unit tests.

  This is done at runtime, which is inefficient, but I can't be bothered
  to write an Autoconf test right now.

  a mapping from the hash to a url has been registered through `nix
  regurl'.

* Bug fix in nix: don't pollute stdout when running tar, it made
  nix-switch barf.

* Bug fix in nix-push-prebuilts: don't create a subdirectory on the
  target when rsync'ing.

  sharing package directories (i.e., the result of building a Nix
  descriptor).

  `nix-pull-prebuilts' obtains a list of all known prebuilts by
  consulting the paths and URLs specified in
  $prefix/etc/nix/prebuilts.conf.  The mappings ($pkghash,
  $prebuilthash) and ($prebuilthash, $location) are registered with
  Nix so that it can use the prebuilt with hash $prebuilthash when
  installing a package with hash $pkghash by downloading and unpacking
  $location.

  `nix-push-prebuilts' creates prebuilts for all packages for which no
  prebuilt is known to exist.  It can then optionally upload these
  to the network through rsync.

  `nix-[pull|push]-prebuilts' just provide a policy.  Nix provides the
  mechanism through the `nix [export|regprebuilt|regurl]' commands.

* Conditionals and variables in Fix expressions.  This allows, e.g.,

    Descr(
    [ Bind("pkgId", "subversion-0.21.0")

    , Bind("httpsClient", Bool(True))
    , Bind("httpServer", Bool(True))

    , Bind("ssl", If(Var("httpsClient"), Fix("./openssl-0.9.7b.fix"), ""))

    , Bind("httpd", If(Var("httpServer"), Fix("./httpd-2.0.45.fix"), ""))
    ...
    ])

  which introduces domain feature variables httpsClient and httpServer
  (i.e., whether Subversion is built with https client and webdav
  server support); the values of the variables influences package
  dependencies and the build scripts.

  The next step is to allow that packages can express constraints on
  each other.  E.g., StrategoXT is dependent on an ATerm library with
  the "gcc" variant enabled.  In fact, this may cause several
  Nix instantiations to be created from a single Fix descriptor.  If
  possible, Fix should try to find the least set of instantiations
  that obeys the constraints.

  package as a root of the garbage collector, unless `--keep' is
  specified.