Merge from CheriBSD:

commit 1d1b908
Author: Brooks Davis <[email protected]>
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 a9d9aa3
Author: Brooks Davis <[email protected]>
Date:   Tue Apr 29 16:06:12 2014 +0000

    Simplify seatbelt added in 1d1b908 based in feedback.

    Discussed with:	[email protected]

Reviewed by:	imp
Sponsored by:	DARPA, AFRL

Makefile

@@ -238,8 +238,17 @@ tinderbox toolchains kernel-toolchains: .MAKE
 ${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