The problem is, if we *do* define "unix", perfectly reasonable,
strictly conforming C programs,  like
	main() { int unix = 0; return unix; }
fail to compile.

On the other hand, if we *don't*, other things fail to compile.

This is a hard problem.  Standard C guarantees I can create functions named
end, etext, read, write, and close, and refer to them from other modules,
and expect it to work.  This is hard to do correctly under Unix.  (Just
recently, I got bitten by naming an external function "end".)

-s