About 3 years ago when I added this stuff to make - so that I could
build the netbsd tree using MAKEOBJDIRPREFIX, it seemed the path of
least resistance...

Today, the tree is regularly built with MAKEOBJDIRPREFIX (build.sh -M)
and I suspect that this logic is no longer needed.

I don't have a machine handy to build -current on, so would appreciate
it if someone who uses build.sh -M could try doing so with
NOCHECKMAKECHDIR set in env or /etc/mk.conf

If the tree builds ok, then I'd propose removing this code.

Thanks
--sjg