Move logic to decide if an MPM is supported, and whether the MPM is

threaded, down to the MPM itself.
  (server/mpm/FOO/config.m4, which runs before the actual MPM selection)

server/mpm/config.m4 makes some general platform checks that can be used
for MPM decisions, and contains some functions related to MPMs.

  XXX The check here for whether APR_POLLSET_THREADSAFE is available
      is a rough approximation and needs to be replaced by a run-time
      check.

Replace the limited per-platform hard-coded MPM selection and the
current defaulting to event (whether or not it works) with a selection
based on which MPMs work on the platform, as reported by the MPMs 
themselves.
  (config2.m4, which runs after the MPMs record whether they are supported)

  Order of preference:

  WinNT (mingw32 only)
    then Event
      then Worker
        then Prefork



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@766082 13f79535-47bb-0310-9956-ffa450edef68