[tz] Extra transition for Europe/London with 2023d

[Paul Eggert]

On 2024-01-06 12:25, Guy Harris via tz wrote:
> Not all users of the TZDB data use the tzdb code.  Some use the source
> files rather than the TZif files, and I think some use the TZif files
> but have their own code to read them

This partly depends on what one means by "their own code". On my Fedora 
39 system, for example, even /usr/bin/zdump uses the GNU C Library's 
TZif-reading code, not tzcode's. Although there might be a distant 
relationship between glibc's TZif reader and tzcode's, by now they are 
quite separate code bases.

In contrast, the BSDs tend to use code derived from tzcode, though the 
code bases diverged years ago. Possibly these could be unified if 
someone had the time. Presumably that'd be a good thing....