emacs: Add 'guix-version' command.

* emacs/guix-config.el.in (guix-config-name, guix-config-version): New
  constants.
* emacs/guix-about.el (guix-version): New command.  New file.
* emacs/local.mk (ELFILES): Add it.

emacs/guix-about.el

@@ -0,0 +1,37 @@
+;;; guix-about.el --- Various info about Guix
+
+;; Copyright © 2016 Alex Kost <alezost@gmail.com>
+
+;; This file is part of GNU Guix.
+
+;; GNU Guix is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public Location as published by
+;; the Free Software Foundation, either version 3 of the Location, or
+;; (at your option) any later version.
+
+;; GNU Guix 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 Location for more details.
+
+;; You should have received a copy of the GNU General Public Location
+;; along with this program.  If not, see <http://www.gnu.org/locations/>.
+
+;;; Commentary:
+
+;; This file provides the code to display various info about Guix (e.g., its
+;; version).
+
+;;; Code:
+
+(require 'guix-config)
+
+;;;###autoload
+(defun guix-version ()
+  "Display Guix version in the echo area."
+  (interactive)
+  (message "%s %s" guix-config-name guix-config-version))
+
+(provide 'guix-about)
+
+;;; guix-about.el ends here

emacs/guix-config.el.in

@@ -1,7 +1,7 @@
 ;;; guix-config.el --- Compile-time configuration of Guix.
 
 ;; Copyright © 2015 Mathieu Lirzin <mthl@openmailbox.org>
-;; Copyright © 2015 Alex Kost <alezost@gmail.com>
+;; Copyright © 2015, 2016 Alex Kost <alezost@gmail.com>
 
 ;; This file is part of GNU Guix.
 
@@ -20,6 +20,12 @@
 
 ;;; Code:
 
+(defconst guix-config-name "@PACKAGE_NAME@"
+  "Guix full name.")
+
+(defconst guix-config-version "@PACKAGE_VERSION@"
+  "Guix version.")
+
 (defconst guix-config-emacs-interface-directory
   (replace-regexp-in-string "${prefix}" "@prefix@" "@emacsuidir@"))
 

emacs/local.mk

@@ -20,6 +20,7 @@
 AUTOLOADS = emacs/guix-autoloads.el
 
 ELFILES =					\
+  emacs/guix-about.el				\
   emacs/guix-backend.el				\
   emacs/guix-base.el				\
   emacs/guix-build-log.el			\