gnu: libgcrypt: Make build reproducible.

* gnu/packages/patches/libgcrypt-make-yat2m-reproducible.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.
* gnu/packages/gnupg.scm (libgcrypt)[source]: Use it.

Signed-off-by: Leo Famulari <leo@famulari.name>

gnu/local.mk

@@ -860,6 +860,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/libevent-2.1-skip-failing-test.patch	\
   %D%/packages/patches/libexif-CVE-2016-6328.patch		\
   %D%/packages/patches/libexif-CVE-2017-7544.patch		\
+  %D%/packages/patches/libgcrypt-make-yat2m-reproducible.patch	\
   %D%/packages/patches/libgit2-mtime-0.patch			\
   %D%/packages/patches/libgdata-fix-tests.patch			\
   %D%/packages/patches/libgdata-glib-duplicate-tests.patch	\

gnu/packages/gnupg.scm

@@ -101,7 +101,9 @@ (define-public libgcrypt
                                  version ".tar.bz2"))
              (sha256
               (base32
-               "0z5gs1khzyknyfjr19k8gk4q148s6q987ya85cpn0iv70fz91v36"))))
+               "0z5gs1khzyknyfjr19k8gk4q148s6q987ya85cpn0iv70fz91v36"))
+             (patches
+              (search-patches "libgcrypt-make-yat2m-reproducible.patch"))))
     (build-system gnu-build-system)
     (propagated-inputs
      `(("libgpg-error-host" ,libgpg-error)))

gnu/packages/patches/libgcrypt-make-yat2m-reproducible.patch

@@ -0,0 +1,32 @@
+Make yat2m in libgcrypt respect SOURCE_DATE_EPOCH, making
+the build reproducible.
+
+This was already fixed upstream in GnuPG:
+https://dev.gnupg.org/rG139de02b93773615bdd95e04a7f0c1ad73b4f6fb
+
+and in libgpg-error:
+https://dev.gnupg.org/rE5494a5728418938d2e42158bb646b07124184e64
+
+
+--- a/doc/yat2m.c 2017-11-23 19:16:58.000000000 +0100
++++ b/doc/yat2m.c 2017-08-28 12:22:54.000000000 +0200
+@@ -1475,6 +1484,7 @@
+ main (int argc, char **argv)
+ {
+   int last_argc = -1;
++  const char *s;
+ 
+   opt_source = "GNU";
+   opt_release = "";
+@@ -1608,6 +1618,11 @@
+   if (argc > 1)
+     die ("usage: " PGM " [OPTION] [FILE] (try --help for more information)\n");
+ 
++  /* Take care of supplied timestamp for reproducible builds.  See
++   * https://reproducible-builds.org/specs/source-date-epoch/  */
++  if (!opt_date && (s = getenv ("SOURCE_DATE_EPOCH")) && *s)
++    opt_date = s;
++
+   /* Start processing. */
+   if (argc && strcmp (*argv, "-"))
+     {