Clean up APR's configure script a bit. This commit also allows Apache to

specify that APR should be built without threads for the prefork MPM.  This
is broken if we then try to configure Apache again using a threaded MPM, but
this is easily fixable, and I will be doing that soon.  Lastly, I removed all
thread calls from APR when it is told to build without threads.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@84359 13f79535-47bb-0310-9956-ffa450edef68