Use ChangeLog date instead of build date

to make builds reproducible.
See https://reproducible-builds.org/ for why this is good.

This date call works with GNU date and (some?) BSD date.

configure.ac

@@ -52,8 +52,8 @@ AC_SUBST(GST_MAJORMINOR)
 dnl release year and date
 DATE_STAMP=`head -n1 "${srcdir}/NEWS" | sed -e 's/^.*(\([[^\)]]*\)).*$/\1/'`
 if test "$DATE_STAMP" == "XX.XXX.XXXX"; then
-  BT_RELEASE_YEAR=`date +%Y`
-  BT_RELEASE_DATE=`date +%Y-%m-%d`
+  BT_RELEASE_YEAR=`date -u -r ChangeLog +%Y`
+  BT_RELEASE_DATE=`date -u -r ChangeLog +%Y-%m-%d`
 else
   IFS="." read -r d m y <<< "$DATE_STAMP"
   BT_RELEASE_YEAR="$y"