nilfs-utils: compile legacy {mount,umount}.nilfs2 with libselinux

SELinux context mount options were optional for the legacy version of
{mount,umount}.nilfs2.  This changes the default build configuration
so that they are built with selinux library unless --without-selinux
options is specified.  When we compile them with libmount, libselinux
is not used regardless of this because context mount options are
provided by libmount.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>