[exim-dev] build-farm / macOS

For awareness,

I've applied on behalf of Exim to
<https://www.macstadium.com/opensource/> to get a free VM to be used as
a build animal.

If we're approved, we'll get rote paperwork every six months to confirm
that we're actually still using it.

I'd like to get macOS/Darwin builds back on the officially supported
list.

Regards,
-Phil