Follow-up to r1523503: Fix BSD build.
If APR has no threading support at all, there won't be thread pools.

* subversion/svnserve/svnserve.c
  (HAVE_THREADPOOLS): define as 0 if APR does not support

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1523572 13f79535-47bb-0310-9956-ffa450edef68