The configure succeeded after upgrading libXinerama to version 1.1. The virtualbox-ose ebuild should have a version dependency on libXinerama. Reproducible: Always Steps to Reproduce:
Created attachment 233001 [details] the configure log from /var/tmp/portage
Yet another on the 1.7 upgrade list. Resync first (see bug 320907). If you are mixing, it's sort of INVALID - in unstable you should already be on libXinerama-1.1. If you have masked 1.7 xserver, then it's still sort of invalid.