stat: avoid compilation failure on AIX 7.x
* src/stat.c (USE_STATVFS): Adjust definition so that it is enabled
also on AIX 7.x systems that provide statvfs64 and no statvfs.
[USE_STATVFS && ! STAT_STATVFS && STAT_STATVFS64] (STATFS): Define
to statvfs64 in that precise case.
* m4/stat-prog.m4 (cu_PREREQ_STAT_PROG): Adjust the condition
here to match the new one in stat.c, to keep them in sync.
Reported by Bruno Haible. For details, see
http://article.gmane.org/gmane.comp.gnu.coreutils.general/1668