As per discussions on current@, protect unsuspecting users from

trying to upgrade their system with make world instead of following
the preferred and suggested sequence of commands. The fact remains
that make world does not upgrade the kernel.
Allow make world when DESTDIR has been specified, including when
DESTDIR specifies the root file system. Otherwise, print a useful
warning and fail.

Reviewed, tested and scrutinized by: gad@

Makefile

@@ -144,14 +144,17 @@ ${TGTS}:
 # Set a reasonable default
 .MAIN:	all
 
+.if defined(DESTDIR)
 .if make(world)
 STARTTIME!= LC_ALL=C date
 .endif
 #
 # world
 #
-# Attempt to rebuild and reinstall *everything*, with reasonable chance of
-# success, regardless of how old your existing system is.
+# Attempt to rebuild and reinstall everything. This target is not to be
+# used for upgrading an existing FreeBSD system, because the kernel is
+# not included. One can argue that this target doesn't build everything
+# then.
 #
 world: upgrade_checks
 	@echo "--------------------------------------------------------------"
@@ -178,6 +181,17 @@ world: upgrade_checks
 	@echo ">>> make world completed on `LC_ALL=C date`"
 	@echo "                   (started ${STARTTIME})"
 	@echo "--------------------------------------------------------------"
+.else
+world:
+	@echo "WARNING: make world will overwrite your existing FreeBSD"
+	@echo "installation without also building and installing a new"
+	@echo "kernel.  This can be dangerous.  Please read the handbook,"
+	@echo "'Using make world', for how to upgrade your system."
+	@echo "Define DESTDIR to the where you want to install FreeBSD,
+	@echo "including /, to override this warning and proceed as usual."
+	@echo "Bailing out now..."
+	@false
+.endif
 
 #
 # kernel