Unable to build world someplace not /usr/obj

[Lee Brown]

I have been trying to buildworld with the following, but it always faults
on contrib/tcsh/sh.c (not related)

make -C /scratch/11.1-releng/src MAKEOBJDIRPREFIX=/scratch/11.1-releng/obj
buildworld

If I just remove the variable, it works fine, except the files don't go
where I want of course.

make -C /scratch/11.1-releng/src buildworld

The build machine is 11.1-RELEASE (r321309) and the source is the same
version.

Can anybody clue me into what I'm doing wrong?  I'm sure I used to do this
no problem.  There are no packages installed, just a vanilla system.

Thanks in advance -- lee