pull: Install (guix config) module to override the user's one.

* build-aux/build-self.scm (zlib, gzip, bzip2, xz): New variables.
(build)[storedir, localstatedir, sysconfdir, sbindir]: New variables.
[builder]: Pass them to 'build-guix'.
* guix/build/pull.scm (build-guix): Add #:system, #:storedir,
  #:localstatedir, #:sysconfdir, #:sbindir, #:package-name,
  #:package-version, #:bug-report-address, #:home-page-url, #:libgcrypt,
  #:zlib, #:gzip, #:bzip2, and #:xz.  Remove #:gcrypt.
  Instantiate all the substitution variables in (guix config).  Remove
  code to delete OUT/guix/config.{scm,go}.
* guix/config.scm.in: Add note about (guix script pull).

build-aux/build-self.scm

@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2014 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2014, 2016 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -19,6 +19,7 @@
 (define-module (build-self)
   #:use-module (gnu)
   #:use-module (guix)
+  #:use-module (guix config)
   #:use-module (srfi srfi-1)
   #:export (build))
 
@@ -44,6 +45,18 @@
 (define libgcrypt
   (first (find-best-packages-by-name "libgcrypt" #f)))
 
+(define zlib
+  (first (find-best-packages-by-name "zlib" #f)))
+
+(define gzip
+  (first (find-best-packages-by-name "gzip" #f)))
+
+(define bzip2
+  (first (find-best-packages-by-name "bzip2" #f)))
+
+(define xz
+  (first (find-best-packages-by-name "xz" #f)))
+
 (define guile-json
   (first (find-best-packages-by-name "guile-json" #f)))
 
@@ -63,6 +76,19 @@
                 #:rest rest)
   "Return a derivation that unpacks SOURCE into STORE and compiles Scheme
 files."
+  ;; The '%xxxdir' variables were added to (guix config) in July 2016 so we
+  ;; cannot assume that they are defined.  Try to guess their value when
+  ;; they're undefined (XXX: we get an incorrect guess when environment
+  ;; variables such as 'NIX_STATE_DIR' are defined!).
+  (define storedir
+    (if (defined? '%storedir) %storedir %store-directory))
+  (define localstatedir
+    (if (defined? '%localstatedir) %localstatedir (dirname %state-directory)))
+  (define sysconfdir
+    (if (defined? '%sysconfdir) %sysconfdir (dirname %config-directory)))
+  (define sbindir
+    (if (defined? '%sbindir) %sbindir (dirname %guix-register-program)))
+
   (define builder
     #~(begin
         (use-modules (guix build pull))
@@ -73,12 +99,28 @@ files."
 
         (build-guix #$output #$source
 
+                    #:system #$%system
+                    #:storedir #$storedir
+                    #:localstatedir #$localstatedir
+                    #:sysconfdir #$sysconfdir
+                    #:sbindir #$sbindir
+
+                    #:package-name #$%guix-package-name
+                    #:package-version #$%guix-version
+                    #:bug-report-address #$%guix-bug-report-address
+                    #:home-page-url #$%guix-home-page-url
+
+                    #:libgcrypt #$libgcrypt
+                    #:zlib #$zlib
+                    #:gzip #$gzip
+                    #:bzip2 #$bzip2
+                    #:xz #$xz
+
                     ;; XXX: This is not perfect, enabling VERBOSE? means
                     ;; building a different derivation.
                     #:debug-port (if #$verbose?
                                      (current-error-port)
-                                     (%make-void-port "w"))
-                    #:gcrypt #$libgcrypt)))
+                                     (%make-void-port "w")))))
 
   (gexp->derivation "guix-latest" builder
                     #:modules '((guix build pull)