GNUmakefile: Require the user to specify '-R' if their make(1) is too old

GNUmakefile: Require the user to specify '-R' if their make(1) is too old

And everyone's make(1) is too old.  :-)

This will allow us to use ?= assignments.  Once a new GNU make(1)
release is done, we'll be able to rely on our setting of MAKEFLAGS+=-R
at the top of the GNUMakefile, but currently, that's not enough, and the
user must specify -R to unset implicit variables.

Cc: Sam James <sam@gentoo.org>
Cc: Paul Smith <psmith@gnu.org>
Cc: Guenther Noack <gnoack@google.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>