[PATCH 1/1] Y2038: add function __difftime64

[Paul Eggert]

Bruno Haible wrote:

>    3) During the next source-code sync from glibc to gnulib, involving mktime.c
>       etc., the gnulib people (likely Paul) make sure that this source code can
>       still be used on non-glibc platforms with either 32-bit time_t or 64-bit
>       time_t. Usually this involves a couple of #ifs and conditional #includes.
>       These changes can then flow back into glibc. They won't have a functional
>       effect in glibc, therefore won't break the atomicity of step 1.

What I'm trying to do is to lessen the work in step (3) by reviewing the changes 
in earlier steps to make (3) simple. Albert had already drafted steps (1) and 
(2) but did so in ways that duplicated code and would have complicated (3).