libmount can propagate device IDs provided by Linux in
/proc/self/mountinfo. However there are currently many
shared libs dependencies introduced by libmount with
associated runtime and virt mem overhead. Therefore don't
enable by default.
* m4/ls-mntd-fs.m4: Use --with-libmount to enable at build time.
Note the ac_cv_lib_libmount_mnt_table_parse_stream cache variable
had a typo and so was ineffective, thus there is no backwards
compatibility issue.