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.

README

@@ -19,7 +19,7 @@ Guix currently depends on the following packages:
   - [[http://nixos.org/nix/][Nix]]
   - [[http://gnupg.org/][GNU libgcrypt]], or [[http://nongnu.org/libchop/][libchop]]
 
-For bootstrapping purposes, it is useful to reuse packages from Nixpkgs.
+Optionally, packages from Nixpkgs may be transparently reused from Guix.
 For this to work, you need to have a checkout of the Nixpkgs repository;
 the `--with-nixpkgs' option allows you to let `configure' know where the
 Nixpkgs checkout is.