manual: Complete @standards in creature.texi.

[Zack Weinberg]

On Fri, Jun 16, 2017 at 8:24 AM, Florian Weimer <fweimer@redhat.com> wrote:
> On 06/16/2017 01:09 PM, Rical Jasan wrote:
>>       * creature.texi (_THREAD_SAFE): Annotate as obsolete.
>>       (_REENTRANT): Likewise.
>
> Content-wise, this looks correct to me (the macros are obsolete).

And as far as I was able to determine when I made them aliases for
_POSIX_C_SOURCE=199506L, they were never part of any standard we track
conformance with, either, so I also endorse this change.

zw