Various System V Interface Definition editions, as well as the X/Open
group portability guide issue 4, recommend defining _XOPEN_SOURCE and
broadly intimating it means the same thing as _POSIX_SOURCE == 2.
Starting in X/Open issue 5 (1995), _XOPEN_SOURCE needs to be defined to
be 500 to bring in the newer interfaces. However, it is still common hat
sources define _XOPEN_SOURCE to be blank. To deal with that, we subtract
0 from _XOPEN_SOURCE to make the other expressions well formed.
While here, define _POSIX_C_SOURCE to be 199209 based on the SVID, the
first version of the Single Unix Specification, and X/Open CAE issue 4,
version 2.
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