MFC: r265097

MFC: r265097

Merge from CheriBSD:

commit 1d1b908107255ffdff4d17f015d8f057d73cc6cb
Author: Brooks Davis <brooks@one-eyed-alien.net>
Date:   Fri Mar 28 16:24:45 2014 +0000

    Add a long needed seatbelt.

    Exit with an error when make is called without a target at the top level
    rather than poluting the source tree and causing use confusion in future
    builds.

commit a9d9aa341b2f4308a227ab460ba85f1f287ad028
Author: Brooks Davis <brooks@one-eyed-alien.net>
Date:   Tue Apr 29 16:06:12 2014 +0000

    Simplify seatbelt added in 1d1b908 based in feedback.

    Discussed with:     imp@FreeBSD.org

Reviewed by:	imp
Sponsored by:	DARPA, AFRL

git-svn-id: svn://svn.freebsd.org/base/stable/9@265450 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f