universe: allow moving build logs to UNIVERSE_LOGDIR

[siva]

Summary

This allows make universe or make tinderbox
to build from a read-only src tree.

[jhb]

I actually find it really convenient that the log files are in .CURDIR.

[siva]

I think the convenience is nice for sure, but the flexibility of choice is reduced. If anything, maybe I can add a variable similar to FAILFILE that is configurable so that users can choose where it goes?

I do like the idea of being able to build from a read-only src tree.

[siva]

How about this instead? This keeps the old behaviour by default
for backwards compatibility.

[delphij]

Looks fine to me.

[jhb]

My only suggestion would be to be consistent and apply this to make universe as well. That is, I would call this UNIVERSE_LOGDIR and move it out of the DOING_TINDERBOX check. This should also be documented in build(7), but I like the idea overall.

[siva]

Thanks, that makes more sense. Updated the diff now, and also removed an unnecessary check against DOING_TINDERBOX because rm -f succeeds when the file doesn't exist anyway.

[siva]

Added documentation in build(7).

[ziaee]

manpage lgtm

[lwhsu]

Other than bumping .Dd of the manpage, LGTM.