configure.in: add --disable-libblkid option

configure.in: add --disable-libblkid option

Add an option to switch between the private (in-tree) libblkid and
public (in-system installed) library.  The private version is still
enabled by default.

If --disable-libblkid is specified the findfs(8) program, which is a
variant of tune2fs, is also not built or installed.

Signed-off-by: Karel Zak <kzak@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>