Add a new directive EnableExceptionHook that must be specified for

exception hooks to be called (in addition to the build time
requirements).

The 2.1-dev feature is now more aligned with the 1.3.30-dev feature,
in that there is a build-time requirement as well as a configuration
requirement.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@102686 13f79535-47bb-0310-9956-ffa450edef68