[tz] file asia needs Link for Asia/Hanoi, dito Europe/Belfast

[Paul Eggert]

Alois Treindl wrote:

> I think file asia should have a line
> 
> Link Asia/Bangkok Asia/Hanoi
> 
> If this is absent, and file backzone is not compiled, no binary file for Hanoi gets created.
> This breaks all references to it.
> 
> For all other zones in backzone, a Link line exists so that a binary file is always created.

That's OK, as there's no requirement that backzone can contain only names that 
are elsewhere in tzdb. In theory we could add more data to backzone for other 
locations. (In practice, I'd rather not bother as this stuff is out of scope and 
we're busy enough as it is.)

> I think the same problem exists for Europe/Belfast
> A Link line for it is in file backwards, but this is not compiled by default, so 
> that no binary Europe/Belfast gets created.

Similarly here. (Europe/Belfast is in backwards because, unlike Asia/Hanoi, it 
formerly was in the main data.)