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"