Make the build reproducible

Whilst working on the Reproducible Builds effort [0] we noticed
that libnbd could not be built reproducibly.

This is due to it shipping a pod generation wrapper that
does not use/respect SOURCE_DATE_EPOCH [1] and additionally
varies the output depending on the build user's current
timezone.

(This was originally filed in Debian as #939546 [2].)

 [0] https://reproducible-builds.org/
 [1] https://reproducible-builds.org/docs/source-date-epoch/
 [2] https://bugs.debian.org/939546

Signed-off-by: Chris Lamb <lamby@debian.org>

podwrapper.pl.in

@@ -217,7 +217,7 @@ if (!$date && -d $filename) {
     $date = $1 if /^(\d+-\d+-\d+)\s/;
 }
 if (!$date) {
-    my ($day, $month, $year) = (localtime)[3,4,5];
+    my ($day, $month, $year) = (gmtime($ENV{SOURCE_DATE_EPOCH} || time))[3,4,5];
     $date = sprintf ("%04d-%02d-%02d", $year+1900, $month+1, $day);
 }