This is ridiculous.  Take Mozilla as an example; xp_core.h does:

#if defined(__unix) || defined(unix) || defined(UNIX) || defined(XP_UNIX)
#ifndef unix
#define unix 1
#endif
#endif

Even though it compiles with -DXP_UNIX, and the conditional ultimately
evaluates true, I *still* get a warning on every file that uses
xp_core.h.

Many packages off the net experience similar problems.