gnu: emacs: Honor 'SOURCE_DATE_EPOCH'.

Suggested by Ludovic Courtès <ludo@gnu.org>.

* gnu/packages/patches/emacs-source-date-epoch.patch: New patch.
* gnu-system.am (dist_patch_DATA): Add it.
* gnu/packages/emacs.scm (emacs)[source]: Use it.

gnu-system.am

@@ -437,6 +437,7 @@ dist_patch_DATA =						\
   gnu/packages/patches/duplicity-test_selection-tmp.patch	\
   gnu/packages/patches/elfutils-tests-ptrace.patch		\
   gnu/packages/patches/emacs-exec-path.patch			\
+  gnu/packages/patches/emacs-source-date-epoch.patch		\
   gnu/packages/patches/eudev-rules-directory.patch		\
   gnu/packages/patches/expat-CVE-2015-1283.patch		\
   gnu/packages/patches/fastcap-mulGlobal.patch			\

gnu/packages/emacs.scm

@@ -70,7 +70,8 @@
              (sha256
               (base32
                "0kn3rzm91qiswi0cql89kbv6mqn27rwsyjfb8xmwy9m5s8fxfiyx"))
-             (patches (list (search-patch "emacs-exec-path.patch")))))
+             (patches (list (search-patch "emacs-exec-path.patch")
+                            (search-patch "emacs-source-date-epoch.patch")))))
     (build-system glib-or-gtk-build-system)
     (arguments
      '(#:phases (alist-cons-before

gnu/packages/patches/emacs-source-date-epoch.patch

@@ -0,0 +1,20 @@
+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.