As discussed in the linked forum post, webapp-config should be an optional dependency, conditional on the 'cgi' USE flag. Patch incoming.
Created attachment 169332 [details, diff] patch to use WEBAPP_OPTIONAL in apcupsd
Thanks for digging into this enough to provide a suggested fix. Assigning to ebuild maintainer to take a look.
Fixed in 3.14.5, thanks.
*** Bug 187008 has been marked as a duplicate of this bug. ***