Add a new target to touch the ${.OBJDIR}/release file, which

indicates the 'release' target has run (in order to prevent
subsequent invocations that may clobber original build output).

As is, the 'release' target is a dummy target that does nothing
more than depend on subsequent targets.  Unless 'make obj' is
invoked prior to 'make release', .OBJDIR and .CURDIR will always
be '/usr/src/release' (or wherever /usr/src is located).

When 'make release' invokes 'make real-release' (and subsequent
targets), .OBJDIR is not updated, which still leads to src/ tree
pollution.

While arguably a hack, 'make release' will invoke the original
dummy targets as originally intended, but instead of touching an
empty file (or returing @true), will call a 'release-done' target
that will trigger the behavior that was intended to prevent
a subsequent invocation.

Discussed with:	hrs
MFC after:	3 days
X-MFC-With:	r293173
Sponsored by:	The FreeBSD Foundation

release/Makefile

@@ -281,7 +281,11 @@ ftp: packagesystem
 	cp *.txz MANIFEST ftp
 
 release:	real-release vm-release cloudware-release
-	touch ${.OBJDIR}/${.TARGET}
+	${MAKE} -C ${.CURDIR} ${.MAKEFLAGS} release-done
+	true
+
+release-done:
+	touch release
 
 real-release:
 	${MAKE} -C ${.CURDIR} ${.MAKEFLAGS} obj