Make svnserve compile with pre-1.3 APR Utils again.

Since those don't provide thread pools, use our old detached thread code
for with APRs.

* subversion/svnserve/svnserve.c
  (HAVE_THREADPOOLS,
   THREAD_ERROR_MSG): new APR-version dependent defines
  (main): fall back to ordinary threads if APR does not have thread pools

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1523503 13f79535-47bb-0310-9956-ffa450edef68