Add a 'apache-mpm-shared' option to build all three MPMs as dynamic modules.
If selected, the existing apache-mpm-event, apache-mpm-prefork and
apache-mpm-worker options determine which will be loaded in the default
config file.

Note: if worker is in the mix, the build will simply never build mod_cgi,
regardless of which MPM is the default.

(fhajny)