RUN_DEPENDS with environment

[pbdlists at pinboard.com]

I'm writing a few ports for automating installation of some internal
stuff. For one such port, I need to have dns/bind84 installed. So my
Makefile says among others:

RUN_DEPENDS=    ${NOEXISTENT}:${PORTSDIR}/dns/bind84

Fine sofar, the port is built, but I need the bind84 port to replace the
system BIND, which is done by setting in the environment, before running
make install:

        PORT_REPLACES_BASE_BIND8=               yes
        PORT_REPLACES_BASE_BIND8_INCLUDES=      yes

The user should not need to know about these settings, so I want to set
these in the Makefile for my port, before the bind84 port is
automatically installed, but I just can't figure out how to do this. Just
setting them in my own Makefile does not do the trick, neither does
something like the following work:

pre-fetch:
        export PORT_REPLACES_BASE_BIND8=yes
        export PORT_REPLACES_BASE_BIND8_INCLUDES=yes

Has anybody done this before and found a workable solution?

Kurt