build: Use deterministic dates for generated man pages

Pin the timezone to UTC so that the computed dates are deterministic
and produce deterministic generated man pages.

Closes: #843469
Signed-off-by: Guillem Jover <guillem@debian.org>

debian/changelog

@@ -7,6 +7,9 @@ dpkg (1.18.13) UNRELEASED; urgency=medium
     - Add missing Breaks on old libdpkg-perl to dpkg. Closes: #843429
       Thanks to Sven Joachim <svenjoac@gmx.de>.
     - Ship deb-src-files(5) man page in dpkg-dev.
+  * Build system:
+    - Use deterministic dates for generated man pages. Closes: #843469
+      Thanks to Reiner Herrmann <reiner@reiner-h.de>.
 
  -- Guillem Jover <guillem@debian.org>  Sun, 06 Nov 2016 06:28:11 +0100
 

m4/dpkg-build.m4

@@ -14,7 +14,7 @@ AC_DEFUN([DPKG_BUILD_SHARED_LIBS], [
 # -----------------------
 AC_DEFUN([DPKG_BUILD_RELEASE_DATE], [
   TIMESTAMP=$(PERL=$PERL $srcdir/run-script scripts/dpkg-parsechangelog.pl -l$srcdir/debian/changelog -STimestamp)
-  PACKAGE_RELEASE_DATE=$(date --date="@$TIMESTAMP" +%Y-%m-%d)
+  PACKAGE_RELEASE_DATE=$(date --utc --date="@$TIMESTAMP" +%Y-%m-%d)
   AC_SUBST([PACKAGE_RELEASE_DATE])
 ])# DPKG_BUILD_RELEASE_DATE