build: Set DOT_USER_PROGRAM for Emacs interface.

Suggested by Ludovic Courtès <ludo@gnu.org>.

* configure.ac: Set DOT_USER_PROGRAM variable.
* emacs/guix-config.el.in (guix-config-dot-program): New constant.
* emacs/guix-external.el (guix-dot-program): Use it.

configure.ac

@@ -206,6 +206,7 @@ AC_CONFIG_FILES([pre-inst-env], [chmod +x pre-inst-env])
 AC_CONFIG_FILES([test-env],     [chmod +x test-env])
 
 dnl Emacs interface.
+AC_PATH_PROG([DOT_USER_PROGRAM], [dot], [dot])
 AM_PATH_LISPDIR
 AM_CONDITIONAL([HAVE_EMACS], [test "x$EMACS" != "xno"])
 

emacs/guix-config.el.in

@@ -1,6 +1,7 @@
 ;;; guix-config.el --- Compile-time configuration of Guix.
 
 ;; Copyright © 2015 Mathieu Lirzin <mthl@openmailbox.org>
+;; Copyright © 2015 Alex Kost <alezost@gmail.com>
 
 ;; This file is part of GNU Guix.
 
@@ -35,6 +36,9 @@ strings of the form:
 
 Where ARGS is a list of arguments to the guile program.")
 
+(defconst guix-config-dot-program "@DOT_USER_PROGRAM@"
+  "Name of the 'dot' executable defined at configure time.")
+
 (provide 'guix-config)
 
 ;;; guix-config.el ends here

emacs/guix-external.el

@@ -23,11 +23,16 @@
 
 ;;; Code:
 
+(require 'guix-config)
+
 (defgroup guix-external nil
   "Settings for external programs."
   :group 'guix)
 
-(defcustom guix-dot-program (executable-find "dot")
+(defcustom guix-dot-program
+  (if (file-name-absolute-p guix-config-dot-program)
+      guix-config-dot-program
+    (executable-find "dot"))
   "Name of the 'dot' executable."
   :type 'string
   :group 'guix-external)