Make variables to force non default libraries and includes?

I need to do the equivalent of  "cd /usr/src/cddl/usr.sbin/dtrace; 
make DESTDIR=/mumble all install"

but it pulls in libraries from the base system, which differ slightly 
from those in the source tree.

How can I force it to use /mumble2/include and /mumble2/lib instead of / ?

I can pre-populate /mumble2 using "make buildworld", "make libraries", 
and "make includes" but
I need to be able to do selective builds of just subdirectories after 
that..  I haven't spotted the right way of forcing the use of the 
"--system_root /mumble2" option in the compiles.

I know we do it in 'buildworld' is there a more generic way?

I have been looking in the .mk files but I haven't spotted it so far.