Prevent a collision with the system's '#define errno ...' by guarding the variable declaration.
Prevent a collision with the system's '#define errno ...' by guarding the variable declaration.


include/my_sys.h:
  There are platforms which use '#define errno ...' and then cause a collision with
  'extern int errno;', one example is OpenBSD.
  Rather than treat them explicitly by the 'HAVE_ERRNO_AS_DEFINE', we can check for these
  by using '#ifndef errno'.
  (Backported from 5.0, as the problem was discovered there first.)

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