app-emulation/virtualbox-5.1.22 failes if MAKEOPTS sets '-j' to something > 1 with fatal error: state/cr_statefuncs.h: No such file or directory This appears to be an old longstanding issue: https://forums.gentoo.org/viewtopic-p-7419002.html?sid=88990ae69237b6145642f43bd2703932#7419002 According to https://devmanual.gentoo.org/ebuild-writing/functions/src_compile/building/ the build should either be patched or "emake -j1" should be used.
*** This bug has been marked as a duplicate of bug 505482 ***