[tz] Glibc 2.19 adds support for version 3 tz format

[Paul Eggert]

The GNU C Library version 2.19, released February 8, adds support for 
version-3 format tz binary files.  I thought I'd give this list a 
heads-up, in case other C-library maintainers are lurking and want to 
keep up with the Joneses.

As you may recall, version 3 is a minor extension to the binary format, 
which adds support for some less-common timekeeping practices.  So far, 
the only tz-database timestamps affected by this change are time stamps 
after the year 2037 in America/Godthab with tz data 2013e or later. 
These time stamps cannot be described by a POSIX TZ string, so version 3 
uses the quasi-POSIX string 'WGT3WGST,M3.5.0/-2,M10.5.0/-1'.

The glibc 2.19 announcement is at 
<https://sourceware.org/ml/libc-alpha/2014-02/msg00224.html>.  More 
details about this change can be found at 
<https://sourceware.org/bugzilla/show_bug.cgi?id=15968>.