[OpenBIOS] Allow building on Mac OS X

[Cormac O'Brien]

> I downloaded a cross compiler that was made to make ppc elf binaries. I
> don't think there is an Apple supplied ppc elf cross compiler.

I'd suggest building your own binutils and gcc, it doesn't take too long and
usually works a bit better.

> > (Unless others here think it's OK to depend on bash instead.)
> Come on people, switch to bash. It's better. What would we miss if we did
> switch to bash?

I think there are probably quite a few people on tcsh or zsh. sh is virtually
guaranteed, bash is not.