release.nix: Pass `--with-libgcrypt-prefix' in the `tarball' job.

* release.nix (jobs.tarball)[configureFlags]: Pass
  `--with-libgcrypt-prefix'.

build: Clearly mark Nixpkgs as optional.

* configure.ac: Always show the result of checking for Nixpkgs.  Don't
  warn when Nixpkgs is not found.
* Makefile.am (AM_DISTCHECK_CONFIGURE_FLAGS): Remove `--with-nixpkgs'
  flag.
* guix/utils.scm (%nixpkgs-directory): Use either the compile-time or
  the run-time `NIXPKGS' environment variable.
* release.nix (jobs.tarball, jobs.build): Remove `--with-nixpkgs'
  configure flag.
* README: Mark Nixpkgs as optional.

* distro/packages/databases.scm, distro/packages/guile.scm,
  distro/packages/typesetting.scm: Change uses of `nixpkgs-derivation*'
  to `nixpkgs-derivation', to avoid failing at compile-time.

release.nix: Use `--no-substitutes' in `distro.hello'.

* release.nix (distro.hello): Use `guix-build --no-substitutes', to
  avoid failures due to unavailable stale substitutes on
  hydra.nixos.org.

release.nix: Change `distro.hello' to produce something.

* release.nix (distro.hello)[buildPhase]: Tee the log to $out.  Add a
  `name' attribute; remove `buildInputs' and instead use the full path
  to `guix-build'.

release.nix: Add a `distro.hello' job.

* release.nix (distro.hello): New job.

release.nix: Pre-download the Guile bootstrap tarball.

* release.nix (bootstrap_guile): New variable.
  (build)[preBuild]: New attribute.

release.nix: Set succeed-on-failure and build-out-source-tree.

* release.nix: Define `succeedOnFailure', `keepBuildDirectory', and
  `buildOutOfSourceTree'.
  (build): Inherit them.

release.nix: Add dependency on GNU Texinfo.

* release.nix (tarball)[buildNativeInputs]: Add `texinfo'.

release.nix: Add dependency on GNU libgcrypt.

* release.nix (build)[configureFlags]: Add `--with-libgcrypt-prefix'.

release.nix: Build out of chroot.

* release.nix: Add `preConfigure' and `__noChroot' attributes.

build: Add release.nix.

* release.nix: New file.
* Makefile.am (EXTRA_DIST): Add it.