It would seem that the test in acinclude.m4 for a NetBSD 3.0 style
getmntinfo() is wrong. The attached patch fixes this.
From the NetBSD 3.0 man page for getmntinfo(3):
RETURN VALUES
On successful completion, getmntinfo() returns a count of the number of
elements in the array. The pointer to the array is stored into mntbufp.
If an error occurs, zero is returned and the external variable errno is
set to indicate the error. Although the pointer mntbufp will be unmodi-
fied, any information previously returned by getmntinfo() will be lost.