Add a global ENABLE_MODULES setting to make it easy
to build and/or activate all possible modules.

A few modules that are not currently buildable needed a
prereq to be defined so that they will be skipped over
appropriately for reasonable values of ENABLE_MODULES.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1520937 13f79535-47bb-0310-9956-ffa450edef68