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

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