90
You will definitely need OS X somehow. Even if you don't own a Mac, you can still try some alternatives.
I found this small documentation on the net: http://devs.openttd.org/~truebrain/compile-farm/apple-darwin9.txt
This describes exactly what you want. I am interested in it myself, haven't tested it yet though (found it 10 minutes ago). The documentation seems good, though.