emacs: Enable 'guix-build-log-minor-mode' in shell buffers.

* emacs/guix-build-log.el (guix-build-log-minor-mode-activate): New variable.
  (guix-build-log-minor-mode-activate-maybe): New function.
* emacs/guix-init.el: Add it to 'shell-mode-hook'.
* doc/emacs.texi (Emacs Build Log): Mention it.

doc/emacs.texi

@@ -646,13 +646,11 @@ Toggle (show/hide) the bodies of all build phases.
 @end table
 
 There is also @kbd{M-x guix-build-log-minor-mode} which also provides
-the same highlighting.  And as it is a minor mode, it can be enabled in
-any buffer.  For example, if you are building some package in a shell
-buffer (@pxref{Interactive Shell,,, emacs, The GNU Emacs Manual}), you
-may enable @command{guix-build-log-minor-mode} to make it more colorful.
-Guix build output is rather specific, so this new highlighting shouldn't
-conflict with the existing one.  This mode also provides the same key
-bindings as the major mode, but prefixed with @kbd{C-c}.
+the same highlighting and the same key bindings as the major mode, but
+prefixed with @kbd{C-c}.  By default, this minor mode is enabled in
+shell buffers (@pxref{Interactive Shell,,, emacs, The GNU Emacs
+Manual}).  If you don't like it, set
+@code{guix-build-log-minor-mode-activate} to nil.
 
 
 @node Emacs Completions

emacs/guix-build-log.el

@@ -102,6 +102,12 @@
   "Face for the number of seconds for a phase."
   :group 'guix-build-log-faces)
 
+(defcustom guix-build-log-minor-mode-activate t
+  "If non-nil, then `guix-build-log-minor-mode' is automatically
+activated in `shell-mode' buffers."
+  :type 'boolean
+  :group 'guix-build-log)
+
 (defcustom guix-build-log-mode-hook '()
   "Hook run after `guix-build-log-mode' is entered."
   :type 'hook
@@ -346,6 +352,13 @@ programmatically using hooks:
   (when font-lock-mode
     (font-lock-fontify-buffer)))
 
+;;;###autoload
+(defun guix-build-log-minor-mode-activate-maybe ()
+  "Activate `guix-build-log-minor-mode' depending on
+`guix-build-log-minor-mode-activate' variable."
+  (when guix-build-log-minor-mode-activate
+    (guix-build-log-minor-mode)))
+
 ;;;###autoload
 (add-to-list 'auto-mode-alist
              ;; Regexp for log files (usually placed in /var/log/guix/...)

emacs/guix-init.el

@@ -15,5 +15,6 @@ avoid loading autoloads of Emacs packages installed in
   (guix-emacs-load-autoloads 'all))
 
 (add-hook 'scheme-mode-hook 'guix-devel-activate-mode-maybe)
+(add-hook 'shell-mode-hook 'guix-build-log-minor-mode-activate-maybe)
 
 (provide 'guix-init)