The apache interface of app-backup/backuppc-3.2.1-r1 needs the following apache modules: * actions * authn_dbm * authz_dbm * cgi * ext_filter I believe backuppc's ebuild should enforce that by checking for proper APACHE_MODULES flags. Otherwise the user has to recompile apache several times, each one including another apache module as "/etc/init.d/apache2-backuppc start" will complain for the lack of one module per try.
I've done this in the recently committed backuppc-3.3.1. Better late than never =) I can't do it in the 3.3.0 ebuild because it's stable.