Gentoo's Bugzilla –
Attachment 516514 Details for
Bug 645654x11-drivers/xf86-video-virtualbox-5.1.32 (with glibc-2.26-r5)- .../work/VirtualBox-5.1.32/src/VBox/Additions/x11/vboxvideo/vboxvideo.h:56:10: fatal error: bits/sigset.h: No such file or directory