Problem solved: I noticed during a teTeX build on another machine that
the 'essential' "-Dunix" was being passed through CFLAGS, and from
that traced the problem back to /etc/mk.conf and my having:

        COPTS		=-O2 -pipe
        CFLAGS		=-O2 -pipe
        CXXFLAGS	=-O2 -pipe

..instead of:

        COPTS		+=-O2 -pipe
        CFLAGS		+=-O2 -pipe
        CXXFLAGS	+=-O2 -pipe

(I've also noticed that /usr/src/usr.sbin/dhcp will successfully build
now too...)

-- 
Chris Baird,, <abuse@brushtail.apana.org.au>