emacs: Fix guix-guile-program default value.

Fixes <http://bugs.gnu.org/21127>.

* emacs/guix-backend.el: Use guix-config.
  (guix-guile-program): Move to ...
* emacs/guix-config.el.in (guix-guile-program): ... here.  Use Guile
  program file name from compile-time instead of depending on PATH.

emacs/guix-backend.el

@@ -52,6 +52,7 @@
 ;;; Code:
 
 (require 'geiser-mode)
+(require 'guix-config)
 (require 'guix-emacs)
 
 (defvar guix-load-path
@@ -63,15 +64,6 @@
   (expand-file-name "guix-helper.scm" guix-load-path)
   "Auxiliary scheme file for loading.")
 
-(defvar guix-guile-program (or geiser-guile-binary "guile")
-  "Name of the guile executable used for Guix REPL.
-May be either a string (the name of the executable) or a list of
-strings of the form:
-
-  (NAME . ARGS)
-
-Where ARGS is a list of arguments to the guile program.")
-
 
 ;;; REPL
 

emacs/guix-config.el.in

@@ -26,6 +26,15 @@
   ;; This must match `NIX_STATE_DIR' as defined in `daemon.am'.
   (or (getenv "NIX_STATE_DIR") "@guix_localstatedir@/guix"))
 
+(defvar guix-guile-program "@GUILE@"
+  "Name of the guile executable used for Guix REPL.
+May be either a string (the name of the executable) or a list of
+strings of the form:
+
+  (NAME . ARGS)
+
+Where ARGS is a list of arguments to the guile program.")
+
 (provide 'guix-config)
 
 ;;; guix-config.el ends here