Fix MPM event/libserf build problems: If libserf was installed during build,

MPM event was picking up the headers but not linking with libserf, causing
unresolved symbol errors.

Now build MPM event with or without serf support depending on --with-serf
being specified or not.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@954273 13f79535-47bb-0310-9956-ffa450edef68