usd: Convert to using libtool

usd: Convert to using libtool

Turn the libusd library into a libtool object (it has no pthread/LWP
differences, and so can just become a pthreaded library), and include
it as such in butc and the volser. liboafs_usd.la is the libtool
object, libusd.a is preserved as a static object for backwards
compatibility.

Change-Id: I25c3d0f429415527fe529c3d3e5afbad9490ea45
Reviewed-on: http://gerrit.openafs.org/8057
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>