Use UTC datetime for reproducible builds

For reproducible builds, we use last modification time of the CHANGES
file.  This works pretty well, unless the builds are made in different
timezones.

Use UTC option to date command to make the builds reproducible.

configure.ac

@@ -1253,7 +1253,7 @@ AM_CONDITIONAL([HAVE_XELATEX], [test "$XELATEX" != ":" && test "$LATEXMK" != ":"
 # Pull release date from CHANGES file last modification date
 # for reproducible builds
 #
-release_date=`date -r CHANGES +%Y-%m-%d`
+release_date=`date -u -r CHANGES +%Y-%m-%d`
 AC_SUBST([RELEASE_DATE], $release_date)
 
 #