maint: Add Guix build files.

* guix.scm, build-aux/manifest.scm: New files.
* Makefile.am (EXTRA_DIST): Add them.
* README: Mention it.

build-aux/manifest.scm

@@ -0,0 +1,61 @@
+;;; Copyright © 2023 Free Software Foundation, Inc.
+;;;
+;;; This file is part of GNU Guile.
+;;;
+;;; GNU Guile is free software; you can redistribute it and/or modify it
+;;; under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation; either version 3 of the License, or (at
+;;; your option) any later version.
+;;;
+;;; GNU Guile is distributed in the hope that it will be useful, but
+;;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;; GNU General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with GNU Guile.  If not, see <http://www.gnu.org/licenses/>.
+
+;; This file defines a Guix manifest for use by Cuirass, the continuous
+;; integration service running at <https://ci.guix.gnu.org>.
+
+(use-modules (guix)
+             (guix profiles))
+
+(define guile
+  (load "../guix.scm"))
+
+(define* (package->manifest-entry* package system
+                                   #:key target)
+  "Return a manifest entry for PACKAGE on SYSTEM, optionally cross-compiled to
+TARGET."
+  (manifest-entry
+    (inherit (package->manifest-entry package))
+    (name (string-append (package-name package) "." system
+                         (if target
+                             (string-append "." target)
+                             "'")))
+    (item (with-parameters ((%current-system system)
+                            (%current-target-system target))
+            package))))
+
+(define native-builds
+  (manifest
+   (map (lambda (system)
+          (package->manifest-entry* guile system))
+        '("x86_64-linux" "i686-linux"
+          "aarch64-linux" "armhf-linux"
+          "powerpc64le-linux"))))
+
+(define cross-builds
+  (manifest
+   (map (lambda (target)
+          (package->manifest-entry* guile "x86_64-linux"
+                                    #:target target))
+        '("i586-pc-gnu"
+          ;; "arm-linux-gnueabihf"
+          "aarch64-linux-gnu"
+          "riscv64-linux-gnu"
+          "i686-w64-mingw32"
+          "x86_64-linux-gnu"))))
+
+(concatenate-manifests (list native-builds cross-builds))