[bug-gettext] Makevars.template identification

[Karl Berry]

I suggest adding a date-of-last-update or other identification to
Makevars.template.  It would help in discovering when Makevars needs to
be updated.  (Of course, determining that automatically, maybe as part
of gettexize, would be better yet, and more reliable.)

A comment with a link to the version control history of the file seems
like it would be useful too.  Namely,
http://git.savannah.gnu.org/cgit/gettext.git/log/gettext-tools/po/Makevars.template

Best,
Karl