Makefile: check that 'run' directory exists.

mdadm default to using /run/mdadm.  However not all distros
provide /run yet.  This can confuse people who build their own
mdadm.
So have "make" complain if the given directory doesn't exist.
This will make it harder to build an mdadm which doesn't work.

Reported-by: Albert Pauw <albert.pauw@gmail.com>
Signed-off-by: NeilBrown <neilb@suse.de>