build.sh ignoring MKUNPRIVED=yes

["Ian D. Leroux" <idleroux%fastmail.fm@localhost>]

I build the base system as a normal user, setting MKUNPRIVED=yes
in /etc/mk.conf to avoid having to specify a command-line option every
time.  As of January first (the last time I rebuilt) this still
worked.  With sources checked out yesterday, however, I see

----------
$ ./build.sh distribution

ERROR: -U or -E must be set for build as an unprivileged user.
*** BUILD ABORTED ***
----------

I get the same error whatever command I give to build.sh (I've tried
cleandir, tools, kernel). If I give the -U option, build.sh works
normally.  I didn't see anything about no longer honouring MKUNPRIVED
in UPDATING.  Was the change deliberate?  Is it documented anywhere?
Or is this something specific to my machine?

Thanks,

--
IDL