pkgsrc cross-compilation

[Taylor R Campbell <campbell+netbsd-tech-pkg%mumble.net@localhost>]

I just committed some changes to improve pkgsrc cross-compilation.  If
you'd like to give it a spin, you can find instructions for using
cross-compilation in pkgsrc/doc/HOWTO-use-crosscompile, and tips for
developing packages for it in pkgsrc/doc/HOWTO-dev-crosscompile.
Eventually these should be merged into the guide, but I haven't done
that yet.

I made sure cross-compilation to powerpc and arm works for me, and I
did a (native) bulk build a little while ago with these changes to
make sure native compilation hasn't broken horribly on amd64, but I'm
sure there remain bugs that I missed.

Please let me know if you have any questions or comments, or if I have
broken anything!