* 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.
