Which version of FreeBSD is this on? Is the system source present in /usr/src?
Welcome to FreeBSD 11.0-CURRENT. Currently there seem to be problems with VirtualBox: https://lists.freebsd.org/pipermail/freebsd-current/2016-March/059909.htmlI have just done a complete re-install of FreeBSD using FreeBSD 11.0, so maybe I shouldn't expect it to build. I installed the ports tree yesterday.