configure: don't link blkid, udev to everything

These are already explicitly called out for libkrbd; don't need them in
LIBS.

Signed-off-by: Sage Weil <sage@redhat.com>
configure.ac: link libboost_thread only with json-spirit

Signed-off-by: Sage Weil <sage@redhat.com>
Merge pull request #2150 from ceph/wip-libs

don't link everything with blkid, udev, and boost_threads