make world for a jail

[Robin Becker]

......
I fixed up my /usr/src tree to be latest RELENG_6_0 in line with my 
machine and ran

cd /usr/src
make world DESTDIR=/usr/jails/mailserver

however, this seems to produce a lot of stuff in /usr/obj (the normal 
make world destination). Eventually the make fails with this message

##########
make: don't know how to make /usr/jails/mailserver/usr/lib/libc.a. Stop
*** Error code 2

Stop in /usr/src.
*** Error code 1

Stop in /usr/src.
##########

In line with my earlier observation that the sub makes were being called 
  like this

make -f Makefile.inc1 DESTDIR=/usr/obj/usr/src/tmp par-all

is there some magic that's required to make world into a specified DESTDIR?
-- 
Robin Becker