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.

Makefile.am

@@ -171,5 +171,5 @@ info_TEXINFOS = doc/guix.texi
 EXTRA_DIST += doc/fdl-1.3.texi
 
 ACLOCAL_AMFLAGS = -I m4
-AM_DISTCHECK_CONFIGURE_FLAGS =					\
-  --with-nixpkgs=$(NIXPKGS) --with-nix-prefix="$(NIX_PREFIX)"
+AM_DISTCHECK_CONFIGURE_FLAGS =			\
+  --with-nix-prefix="$(NIX_PREFIX)"

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.

configure.ac

@@ -52,13 +52,12 @@ AC_ARG_WITH([nixpkgs],
    esac],
   [])
 
+AC_MSG_CHECKING([for Nixpkgs source tree])
 if test -f "$NIXPKGS/default.nix"; then
-   AC_MSG_CHECKING([for Nixpkgs source tree])
    AC_MSG_RESULT([$NIXPKGS])
    AC_SUBST([NIXPKGS])
 else
-   AC_MSG_WARN([Nixpkgs not found; this will prevent most tests from running.])
-   AC_MSG_WARN([Please use `--with-nixpkgs'.])
+   AC_MSG_RESULT([not found])
 fi
 
 AC_ARG_WITH([libgcrypt-prefix],

distro/packages/databases.scm

@@ -41,10 +41,10 @@ (define-public recutils
              (base32
               "1v2xzwwwhc5j5kmvg4sv6baxjpsfqh8ln7ilv4mgb1408rs7xmky"))))
    (build-system gnu-build-system)
-   (inputs `(("curl" ,(nixpkgs-derivation* "curl"))
-             ("emacs" ,(nixpkgs-derivation* "emacs"))
-             ("check" ,(nixpkgs-derivation* "check"))
-             ("bc" ,(nixpkgs-derivation* "bc"))
+   (inputs `(("curl" ,(nixpkgs-derivation "curl"))
+             ("emacs" ,(nixpkgs-derivation "emacs"))
+             ("check" ,(nixpkgs-derivation "check"))
+             ("bc" ,(nixpkgs-derivation "bc"))
              ("patch/gets"
               ,(search-patch "diffutils-gets-undeclared.patch"))))
    (arguments `(#:patches (list (assoc-ref %build-inputs "patch/gets"))))

distro/packages/guile.scm

@@ -45,7 +45,7 @@ (define (guile-reader guile)
                "1svlyk5pm4fsdp2g7n6qffdl6fdggxnlicj0jn9s4lxd63gzxy1n"))))
    (build-system gnu-build-system)
    (native-inputs `(("pkgconfig" ,pkg-config)
-                    ("gperf" ,(nixpkgs-derivation* "gperf"))))
+                    ("gperf" ,(nixpkgs-derivation "gperf"))))
    (inputs `(("guile" ,guile)))
    (description "Guile-Reader, a simple framework for building readers for
 GNU Guile")

distro/packages/typesetting.scm

@@ -94,7 +94,7 @@ (define out
                "12gkyqrn0kaa8xq7sc7v3wm407pz2fxg9ngc75aybhi5z825b9vq"))))
     (build-system gnu-build-system)               ; actually, just a makefile
     (outputs '("out" "doc"))
-    (inputs `(("ghostscript" ,(nixpkgs-derivation* "ghostscript"))))
+    (inputs `(("ghostscript" ,(nixpkgs-derivation "ghostscript"))))
     (arguments `(#:modules ((guix build utils)
                             (guix build gnu-build-system)
                             (srfi srfi-1))        ; we need SRFI-1

guix/utils.scm

@@ -458,7 +458,8 @@ (define sha256
 (define %nixpkgs-directory
   (make-parameter
    ;; Capture the build-time value of $NIXPKGS.
-   (compile-time-value (getenv "NIXPKGS"))))
+   (or (compile-time-value (getenv "NIXPKGS"))
+       (getenv "NIXPKGS"))))
 
 (define* (nixpkgs-derivation attribute #:optional (system (%current-system)))
   "Return the derivation path of ATTRIBUTE in Nixpkgs."

release.nix

@@ -51,7 +51,7 @@ let
         buildInputs = with pkgs; [ guile ];
         buildNativeInputs = with pkgs; [ texinfo gettext cvs pkgconfig ];
         configureFlags =
-          [ "--with-nixpkgs=${nixpkgs}" "--with-nix-prefix=${pkgs.nix}" ];
+          [ "--with-nix-prefix=${pkgs.nix}" ];
       };
 
     build =
@@ -64,7 +64,7 @@ let
         buildNativeInputs = [ pkgs.pkgconfig ];
         src = jobs.tarball;
         configureFlags =
-          [ "--with-nixpkgs=${nixpkgs}" "--with-nix-prefix=${pkgs.nix}"
+          [ "--with-nix-prefix=${pkgs.nix}"
             "--with-libgcrypt-prefix=${pkgs.libgcrypt}"
           ];