Gentoo's Bugzilla –
Attachment 551298 Details for
Bug 668678app-emulation/virtualbox-guest-additions-5.2.18 - /var/tmp/portage/app-emulation/virtualbox-guest-additions-5.2.18/work/vboxguest/Makefile.include.header:99: *** The variable KERN_DIR must be a kernel build folder and end with /build without a trailing