Date:        Mon, 22 Jun 1998 00:41:04 -0400 (EDT)
    From:        woods@most.weird.com (Greg A. Woods)
    Message-ID:  <m0ynyPg-0009QrC@most.weird.com>

  | SysVr2.x for VAX and 3b2 also defined 'unix'.  I sort of remember some
  | variant of 7th Edition doing so as well, ...

The "unix" define is ancient.   Recall that back at Bell Labs they used
C under GCOS on an HIS 6070 computer (whatever that was) (according to my
6th edition C reference manual anyway) as well as on unix (PDP/11) systems.
When #ifdef was added to C, adding "unix" and "gcos" as standard defines to
allow appropriate code to be included had to be the obvious thing to do.

While actually using "unix" as an #if selector these days is almost always
going to be the wrong thing to do, deleting it would be messing about with
a truly ancient tradition - and one upon which lots of programs rely.

kre