Select appropriate PG_PRINTF_ATTRIBUTE for recent NetBSD.

Select appropriate PG_PRINTF_ATTRIBUTE for recent NetBSD.

NetBSD-current generates a large number of warnings about "%m" not
being appropriate to use with *printf functions.  While that's true
for their native printf, it's surely not true for snprintf.c, so I
think they have misunderstood gcc's definition of the "gnu_printf"
archetype.  Nonetheless, choosing "__syslog__" instead silences the
warnings; so teach configure about that.

Since this is only a cosmetic warning issue (and anyway it depends
on previous hacking to be self-consistent), no back-patch.

Discussion: https://postgr.es/m/16785.1539046036@sss.pgh.pa.us