Fix build for separate objdirs

In particular, setting UNTHREADED=${srcdir}/dir and then referring
to ${srcdir}/${UNTHREADED} is a recipe for sadness.

Fix the libtool invocation to correctly find .la.sym files in ${srcdir}.

Also add some missing header dependencies that are hidden when building
in the src tree.

Change-Id: I1b663141c549cc4b90e46327c3fb31dc80294eaa
Reviewed-on: http://gerrit.openafs.org/8891
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>