m4: fix statx override selection if /usr/include doesn't define it

m4: fix statx override selection if /usr/include doesn't define it

If the system headers (aka the ones in /usr/include) do not define
struct statx at all, we need to use our internal override.  The m4 code
doesn't handle this admittedly corner case, but let's fix it for anyone
trying to build new xfsprogs on a decade-old distribution.

Fixes: 409477af604f46 ("xfs_io: add support for atomic write statx fields")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>