Honor SOURCE_DATE_EPOCH variable to avoid non-determinism in generated
"autoloads" files.

--- a/lisp/emacs-lisp/autoload.el
+++ b/lisp/emacs-lisp/autoload.el
@@ -378,8 +378,12 @@
   "Insert the section-header line,
 which lists the file name and which functions are in it, etc."
   (insert generate-autoload-section-header)
-  (prin1 `(autoloads ,autoloads ,load-name ,file ,time)
-	 outbuf)
+  (let* ((env  (getenv "SOURCE_DATE_EPOCH"))
+         (time (if env
+                   (seconds-to-time (string-to-number env))
+                 time)))
+    (prin1 `(autoloads ,autoloads ,load-name ,file ,time)
+           outbuf))
   (terpri outbuf)
   ;; Break that line at spaces, to avoid very long lines.
   ;; Make each sub-line into a comment.
