gnu: emacs: Add lexical binding cookies.

* gnu/packages/emacs.scm (emacs-minimal)[#:phases]<install-c-source>
<install-site-start>: Add “;; -*- lexical-binding: t; -*-” at the top of the
generated files.
* gnu/packages/aux-files/emacs/comp-integrity.el: Likewise.

gnu/packages/aux-files/emacs/comp-integrity.el

@@ -1,3 +1,4 @@
+;; -*- lexical-binding: t; -*-
 (require 'ert)
 
 (eval-when-compile

gnu/packages/emacs.scm

@@ -387,6 +387,7 @@
                   (lambda ()
                     (display
                      (string-append
+                      ";; -*- lexical-binding: t; -*-\n"
                       "(setq find-function-C-source-directory \"" dest "\")\n\n"
                       "(provide 'guix-emacs-c-source)")))))))
           (add-after 'install 'install-site-start
@@ -413,6 +414,7 @@
                   (lambda ()
                     (display
                      (string-append
+                      ";; -*- lexical-binding: t; -*-\n"
                       "(when (require 'guix-emacs nil t)\n"
                       "  (guix-emacs-autoload-packages 'no-reload)\n"
                       "  (advice-add 'package-load-all-descriptors"