nilfs-utils: add --with-libmount build option

The libmount build option of the current configure script is confusing
since it is provided as an --enable-<feature> style option.

This fixes the issue by adding --with-libmount configure option.  The
--enable-libmount option is still available for compatibility reason.

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