looks like it has a naive version test
>>> Configuring source in /var/tmp/portage/x11-drivers/xf86-video-virtualbox-4.3.26/work/VirtualBox-4.3.26 ...
Checking for environment: Determined build machine: linux.amd64, target machine: linux.amd64, OK.
Checking for kBuild: found, OK.
Checking for gcc:
** gcc version 5.0.0-alpha20150322 found, expected gcc 3.x with x>1 or gcc 4.x with 0<x<10!
Check /var/tmp/portage/x11-drivers/xf86-video-virtualbox-4.3.26/work/VirtualBox-4.3.26/configure.log for details
* ERROR: x11-drivers/xf86-video-virtualbox-4.3.26::gentoo failed (configure phase):
Created attachment 402400 [details, diff]
Patch to allow configure to accept gcc version 5.x
Here is a patch that changes the gcc version check to allow gcc 5.x in addition to the other ranges. Virtualbox does compile with gcc 5.1, so this was a naive check.
Fixed in >=app-emulation/virtualbox-4.3.30