Update timezone code from tzcode 2017b

[Stan Shebs]

On Thu, Jun 15, 2017 at 5:32 PM, Joseph Myers <joseph@codesourcery.com> wrote:
>
> This patch updates files coming from tzcode to the versions in tzcode
> 2017b.

So this blows up for me compiling with GCC 4.8.4 (yes yes I know, it's
what Google makes
me use), which doesn't like the _Generic in timezone/private.h.

Does it make sense to try to borrow the __HAVE_GENERIC_SELECTION added
for float128,
or is this more of a sign to start requiring 4.9 as the minimum?

Stan