See attached build log.
(Please do not complain about the need for looking at the build log until you can provide an easy way to open bugs with the correct data picked out of a tinderbox log. Thanks.)
Created attachment 242449 [details]
Should be fixed with x11-drivers/xf86-video-virtualbox-4.0.0
Still not fixed... reopening.
A solution that *should* apparently work is adding
TEMPLATE_VBOXGUESTR3EXE_LDFLAGS += $(LDFLAGS)
to files/xf86-video-virtualbox-3-localconfig (copied to LocalConfig.kmk by the ebuild).
For some reason, however, it doesn't work, and I am not knowledgeable enough about kBuild to find out why.
*** Bug 430586 has been marked as a duplicate of this bug. ***
I don't see this error in any current versions