Allow use of unthreaded Apache
    Allow the use of an Apache without threading support.  The mod_admserv
    authorization cache will not work correctly without threading support.
    This means that most Console and Admin Server tasks will fail if the
    configuration directory server is not available.  If an Apache without
    threading is found, configure will print an error and abort.  You can
    force the use of an unthreaded Apache by using the --disable-threading
    configure option.  Example:
     configure --with-httpd=/usr/sbin/httpd # non-threaded
     ...
     checking for /usr/sbin/httpd with threading support... configure: error: threading model not supported - use --disable-threading to force use of unthreaded model
     configure aborts
    Threading explicitly disabled:
     configure --disable-threading --with-httpd=/usr/sbin/httpd # non-threaded
     ...
     checking for --enable-threading... no
     checking for /usr/sbin/httpd with threading support... NOTICE - threading support explicitly disabled - Admin Server authorization cache will not work correctly
     ...
    configure detects that /usr/sbin/httpd does not use the Worker model (that
    is, threading support) and prints a warning.
    
    Also fix some minor compilation warnings.