nilfs-utils: allow to make without libblkid

Distributions with old blkid library fail to configure
nilfs-utils-2.1.5.

This fixes the issue by making the use of libblkid optional.

Now, user can eliminate dependency to libblkid by specifying
"--without-blkid" option when running the configure script:

 $ ./configure --without-blkid
 $ make

By default, the blkid library is used to mkfs.nilfs2 safer.

Reported-by: Seiji Kihara <kihara.seiji@lab.ntt.co.jp>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>