Multiple/Split Master Sites?

[Jon Buller <jon%Bullers.Net@localhost>]

I'm looking at fixing lang/sbcl... It's a Lisp compiler, written in
Lisp, so it needs a binary to build itself with. I think lang/gcc*-ada
might have similar problems, so it might be next. Currently, you need
to find some binary somewhere else, install it in /usr/local/, /tmp/,
~/, or someplace else accessible, hack the package Makefile to know
where you put the host compiler, and then make the package.

I would like to take an existing binary package, unpack it in the work/
directory and build from there. So the first big question is how can I
have MASTER_SITES set to the right SOURCEFORGE spot to get the sources,
and also fetch the proper binary package, presumably from ftp.n.o or a
mirror. Second, is there any prior art on how to get the right binary
package? I suspect I can muddle through that one, but hints would be
appreciated.

Any other advice before I start really hacking on the
lang/sbcl/Makefile? Thanks in advance,

Jon