[tz] Undoing the effect of the new alike-since-1970 patch

[Paul Eggert]

There's been a clear need expressed to support tzdb users who would 
rather not deal with the effects of the recently-proposed 
alike-since-1970 patch. On the other hand there are also fairness and 
guideline-oriented reasons for the patch, which was originally discussed 
and installed with more than our usual care and review.

Because of the extensive followup discussions I don't see how a single 
version could be appealing to both sides of this disagreement.

So I propose we add a Makefile or similar build-time option to let tzdb 
users have it either way. Set the flag one way, and it will be as if the 
recently-proposed changes did not occur. Set it the other way, and 
you'll get the changes.

This will be similar to our vanguard/rearguard support, which helped 
address a similar disagreement back in 2018. We could even control 
things via the existing vanguard/rearguard build-time option; that might 
be simpler. If you have ideas about what the the option's specific 
effects should be, please let me know.

If there is support for this idea, I expect to be able to implement this 
option soon, in plenty of time before any urgent change due to a 
real-world rule change arrives.

Like most compromises, I expect this approach to not be entirely 
satisfactory to anybody. However, I hope it helps us move forward together.