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:	
Sponsored by:	DARPA, AFRL
---------------------------------------------------------------------
Index: Makefile
===================================================================
--- Makefile	(revision 265059)
+++ Makefile	(working copy)
@@ -238,9 +238,18 @@
 ${TGTS}:
 	${_+_}@cd ${.CURDIR}; ${_MAKE} ${.TARGET}
 
-# Set a reasonable default
-.MAIN:	all
+# The historic default "all" target creates files which may cause stale
+# or (in the cross build case) unlinkable results. Fail with an error
+# when no target is given. The users can explicitly specify "all"
+# if they want the historic behavior.
+.MAIN:	_guard
 
+_guard:
+	@echo
+	@echo "Explicit target required (use \"all\" for historic behavior)"
+	@echo
+	@false
+
 STARTTIME!= LC_ALL=C date
 CHECK_TIME!= find ${.CURDIR}/sys/sys/param.h -mtime -0s ; echo
 .if !empty(CHECK_TIME)