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.
Suggested by Ludovic Courtès <ludo@gnu.org>.
* emacs/guix-external.el (guix-png-file-name): Change prefix of a file
name to "guix-emacs-graph-" to avoid possible conflicts.