Honor SOURCE_DATE_EPOCH when exporting configuration date.
Autoconf-level patch submitted upstream on Wed Apr 13 17:03:23 UTC 2016

--- a/configure
+++ b/configure
@@ -27737,7 +28573,14 @@
 
 
 ## Configuration date
- CONFIG_DATE="`date`"
+ CONFIG_DATE="`date -u`"
+if test -n "$SOURCE_DATE_EPOCH"; then
+  CONFIG_DATE=`date -u -d "@$SOURCE_DATE_EPOCH" 2>/dev/null \
+               || date -u -r "$SOURCE_DATE_EPOCH" 2>/dev/null`
+  if test -z "$CONFIG_DATE"; then
+    as_fn_error $? "malformed SOURCE_DATE_EPOCH" "$LINENO" 5
+  fi
+fi
 
 ## User doing the configuration
  CONFIG_USER="`whoami`@`hostname`"
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<tr><td><span title='2020-06-01 00:51:39 +0200'>2020-06-01</span></td><td><a href='/guix/commit/gnu/packages/patches/biber-sortinithash.patch?id=912be8ec342a04bd4f50c353f2776408fbcfea74'>gnu: biber: Update to 2.12.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-04-24 16:22:02 +0200'>2020-04-24</span></td><td><a href='/guix/commit/gnu/packages/patches/biber-sortinithash.patch?id=40fd909e3ddee2c46a27a4fe92ed49d3e7ffb413'>gnu: biber: Downgrade to 2.11.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-04-24 16:21:13 +0200'>2020-04-24</span></td><td><a href='/guix/commit/gnu/packages/patches/biber-sortinithash.patch?id=122b3cd68dbc6c83de5357d4db4d82d2fd92d9b1'>gnu: biber: Address test failures.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>