stop exporting APACHE_MPM_DIR to modules -- unclear why this was ever necessary, and it

  makes no sense with loadable MPMs
axe the unnecessary MPM_DIR build variable
move the stray APACHE_SUBST(MPM_LIB) from configure.in to server/mpm/config.m4, where
  the other MPM_foos are handled


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@760403 13f79535-47bb-0310-9956-ffa450edef68