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
@@ -419,8 +419,12 @@
   ;;              nil t))
   ;;        (match-end 2))))
   (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.
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<div class='path'>path: <a href='/guix/log/?id=9df26b02064f30ca1b98a3084341c36d0479deea'>root</a>/<a href='/guix/log/gnu?id=9df26b02064f30ca1b98a3084341c36d0479deea'>gnu</a>/<a href='/guix/log/gnu/bootloader.scm?id=9df26b02064f30ca1b98a3084341c36d0479deea'>bootloader.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/gnu/bootloader.scm?id=9df26b02064f30ca1b98a3084341c36d0479deea&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2019-08-30 01:33:45 +0200'>2019-08-30</span></td><td><a href='/guix/commit/gnu/bootloader.scm?id=a28cfee841e9c5ab179291d3065f1486fd065e7e'>system: Add 'bootloader-menu-entries' field to &lt;boot-parameters&gt;.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-08-30 01:33:45 +0200'>2019-08-30</span></td><td><a href='/guix/commit/gnu/bootloader.scm?id=ac9cd78ea9e6a3cf850b33fb2111b85e2bbf533d'>bootloader: Fix comment about 'menu-entries'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-03-24 23:06:11 +0100'>2019-03-24</span></td><td><a href='/guix/commit/gnu/bootloader.scm?id=8d058e7b1b1a409d3d9cc29c5650a98db4e78783'>bootloader: Add a 'keyboard-layout' field.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-03-24 23:06:10 +0100'>2019-03-24</span></td><td><a href='/guix/commit/gnu/bootloader.scm?id=91b6873b24c205bd445844845fb44add643d0cff'>bootloader: Reindent record type definition.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-03-24 23:06:10 +0100'>2019-03-24</span></td><td><a href='/guix/commit/gnu/bootloader.scm?id=8a0e1bb12b3c22a8c9a2be17492058ca63ec7c5d'>bootloader: Remove unused 'additional-configuration' field.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-01-16 14:07:38 +0100'>2019-01-16</span></td><td><a href='/guix/commit/gnu/bootloader.scm?id=5f7467f046c3c1648cdf6eb81b3ec041bfc1f9bb'>bootloader: Remove deprecated 'device' field.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-04-08 17:48:33 +0200'>2018-04-08</span></td><td><a href='/guix/commit/gnu/bootloader.scm?id=3c0128b035ae00462f1b0a4427d9525d750e5575'>discovery: Remove dependency on (guix ui).</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>