This ebuild gives a ton of info about the CGI component of apcupsd. I think it should also suggest that the user enable CONFIG_USB_HIDDEV in his/her kernel when the usb useflag is enabled. Maybe it can detect if the kernel was built with/without CONFIG_USB_HIDDEV. However, this is merely for helping the user. I recognize that the build should not require the presence of kernel sources :-)
Added some warnings on 3.14.7.