rxperf: Add build rules to build a pthreaded version

rxperf: Add build rules to build a pthreaded version

The pthreaded version of rxperf is much more interesting than the
LWP one now, so add some rules so that Unix can play with it too.

Reviewed-on: http://gerrit.openafs.org/2841
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit c21c90515ac37862a6002441790a721489fa89a8)
Change-Id: Ie8c26191e6d7603dcd6a99ef9938efb33dea1e33
Reviewed-on: http://gerrit.openafs.org/2920