[PATCH] Fix Wundef warnings in posix/confstr.c

[Roland McGrath]

I was thinking about this and the similar cases in sysconf.c.  I think we
should do something better about them.  The straightforward approach you've
taken is still scarily typo-prone.  At the very least, we can use a macro
that voids actually typing the _POSIX_FOOBAR name type.  Something even
better is more doable for the sysconf case than the confstr one.  But we
should give it some more thought.

Thanks,
Roland