Build system changes

[Richard Kettlewell]

On 21/10/2021 16:44, Russ Allbery wrote:
> 1. The pluggable storage, overview, and history mechanism, where we
>     generate build rules at build time, is possible with Automake but it
>     would add a fair amount of complexity.  I feel like it's a very
>     uncommon operation to add a new storage, overview, or history backend,
>     and if one is doing this, modifying the top-level Makefile.am is the
>     least part of that work, so we should drop that level of dynamic
>     generation.

This was a big source of confusion while analyzing #206, so I'll be glad 
to see the back of it.

ttfn/rjk