configure: don't fail if /usr/include/sys/mount.h does not exist

configure: don't fail if /usr/include/sys/mount.h does not exist

The AX_CHECK_MOUNT_OPT macro fails if /usr/include/sys/mount.h.
Unfortunately, the GNU HURD doesn't have this header file.  Drop the
requirement, since if it doesn't exist, the macro will assume that
given mount options don't exist, which for e2fsprogs is used to test
to for the existence of the mount options nosuid and nodev.  This is
only used for fuse2fs, and HURD doesn't support fuse2fs anyway.

Addresses-Debian-Bug: #944649
Reported-by: Svante Signell <svante.signell@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>