build: port to AIX

build: port to AIX

Problems reported by Michael Felt, and and part of this fix taken
from code suggested by Pádraig Brady in:
http://bugs.gnu.org/20733#112
* configure.ac (stdbuf_supported): Check for warnings, and
for -fPIC and -shared, for AIX.
* src/stat.c (STRUCT_STATVFS): Define to struct statvfs64 if
STATFS is statvfs64.
* src/sync.c (sync_arg) [_AIX]: Open in write mode,
since AIX fsync doesn't work on read-only file descriptors.
* tests/misc/wc-parallel.sh: Skip test if xargs -P does not work.