[Build] Enabling automatic object directory creation

[Simon J. Gerraty]

Warner Losh <imp at bsdimp.com> wrote:
> > The feature is named 'WITH_AUTO_OBJ'.  Enabling this by default means
> > that the only way to disable it is to add WITHOUT_AUTO_OBJ=yes to
> > environment, make argument or /etc/src-env.conf.
> 
> I tend to think this is a good idea. However, more and more is moving
> into sys.mk, and it's been collecting lots of odd pebbles...

The issue in this case, is that presence/absence of objdir affects
.PATH, so if auto objdir is to be done, it greatly simplifies life to do it
as early as possible ie, during sys.mk