Bail out at configure time if an invalid MPM was specified.

Submitted by: jean-frederic clere <jfrederic.clere@fujitsu-siemens.com>
Reviewed by:  Jeff Trawick


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@92098 13f79535-47bb-0310-9956-ffa450edef68