build: Add dependency on Git.

* configure.ac: Check for ‘git’ and substitute ‘GIT’.
* guix/config.scm.in (%git): New variable.
* guix/self.scm (compiled-guix): Define ‘git’ and pass it to
‘make-config.scm’.
(make-config.scm): Add #:git; emit a ‘%git’ variable.
* doc/guix.texi (Requirements): Add it.

configure.ac

@@ -201,6 +201,13 @@ AC_SUBST([GZIP])
 AC_SUBST([BZIP2])
 AC_SUBST([XZ])
 
+dnl Git is now required for the "builtin:git-download" derivation builder.
+AC_PATH_PROG([GIT], [git])
+if test "x$GIT" = "x"; then
+  AC_MSG_ERROR([Git is missing; please install it.])
+fi
+AC_SUBST([GIT])
+
 LIBGCRYPT_LIBDIR="no"
 LIBGCRYPT_PREFIX="no"
 

doc/guix.texi

@@ -1011,6 +1011,7 @@ version 0.1.0 or later;
 @item
 @uref{https://gitlab.com/guile-git/guile-git, Guile-Git}, version 0.5.0
 or later;
+@item @uref{https://git-scm.com, Git} (yes, both!);
 @item @uref{https://savannah.nongnu.org/projects/guile-json/, Guile-JSON}
 4.3.0 or later;
 @item @url{https://www.gnu.org/software/make/, GNU Make}.

guix/config.scm.in

@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2012, 2013, 2014, 2015, 2016, 2018, 2019, 2021 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2012-2016, 2018-2019, 2021, 2023 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2017 Caleb Ristvedt <caleb.ristvedt@cune.org>
 ;;;
 ;;; This file is part of GNU Guix.
@@ -35,6 +35,7 @@ (define-module (guix config)
             %config-directory
 
             %system
+            %git
             %gzip
             %bzip2
             %xz))
@@ -109,6 +110,9 @@ (define %config-directory
 (define %system
   "@guix_system@")
 
+(define %git
+  "@GIT@")
+
 (define %gzip
   "@GZIP@")
 

guix/self.scm

@@ -69,6 +69,7 @@ (define %packages
       ("gzip"               . ,(ref 'compression 'gzip))
       ("bzip2"              . ,(ref 'compression 'bzip2))
       ("xz"                 . ,(ref 'compression 'xz))
+      ("git-minimal"        . ,(ref 'version-control 'git-minimal))
       ("po4a"               . ,(ref 'gettext 'po4a))
       ("gettext-minimal"    . ,(ref 'gettext 'gettext-minimal))
       ("gcc-toolchain"      . ,(ref 'commencement 'gcc-toolchain))
@@ -826,6 +827,9 @@ (define disarchive
   (define guile-lzma
     (specification->package "guile-lzma"))
 
+  (define git
+    (specification->package "git-minimal"))
+
   (define dependencies
     (append-map transitive-package-dependencies
                 (list guile-gcrypt guile-gnutls guile-git guile-avahi
@@ -999,6 +1003,7 @@ (define *config*
                     => ,(make-config.scm #:gzip gzip
                                          #:bzip2 bzip2
                                          #:xz xz
+                                         #:git git
                                          #:package-name
                                          %guix-package-name
                                          #:package-version
@@ -1104,7 +1109,7 @@ (define %default-config-variables
     (%storedir . "/gnu/store")
     (%sysconfdir . "/etc")))
 
-(define* (make-config.scm #:key gzip xz bzip2
+(define* (make-config.scm #:key gzip xz bzip2 git
                           (package-name "GNU Guix")
                           (package-version "0")
                           (channel-metadata #f)
@@ -1134,6 +1139,7 @@ (define defmod 'define-module)
                                %state-directory
                                %store-database-directory
                                %config-directory
+                               %git
                                %gzip
                                %bzip2
                                %xz))
@@ -1176,6 +1182,8 @@ (define %channel-metadata
                      ;; information is used by (guix describe).
                      '#$channel-metadata)
 
+                   (define %git
+                     #+(and git (file-append git "/bin/git")))
                    (define %gzip
                      #+(and gzip (file-append gzip "/bin/gzip")))
                    (define %bzip2