bug#21020: 24.4; `display-time-world' tampers with TZ

[Eli Zaretskii]

> Date: Sun, 26 Jul 2015 00:09:40 -0700
> From: Paul Eggert <eggert@cs.ucla.edu>
> CC: 21020@debbugs.gnu.org, Eli Zaretskii <eliz@gnu.org>
> 
> Thanks for the bug report.  Improving Emacs time zones has been on my TODO 
> list 
> for ages, and the report prompted me to do that.  Attached is a patch which 
> works for me on GNU/Linux.  I'm CC'ing this to Eli to give him a heads-up in 
> case it has implications on MS-Windows.

This is a large patch, touching a lot of code.  I hope we have some
tests to make sure this doesn't cause any regressions.

Other than that, please make in nt/gnulib.mk the same changes you did
in lib/gnulib.mk, otherwise I think the changes will not compile on
Windows.

Thanks.