Gentoo's Bugzilla –
Attachment 356544 Details for
Bug 424816app-emulation/virtualbox-guest-additions-4.1.12 and x11-drivers/xf86-video-virtualbox-4.1.12 with a separate kernel build directory - .../work/VirtualBox-4.1.12/include/iprt/types.h:104:29: fatal error: linux/version.h: No such file or directory