In for loop values, introduce a dummy to protect against empty list

In constructions such as 'for x in $(MAKEVAR); do ...', there's the
possibility that $(MAKEVAR) is en empty value.  Some shells don't like
that, so introduce a dummy value that gets discarded:

    for x in dummy $(MAKEVAR); do
        if [ "$$x" = "dummy" ]; then continue; fi

Closes RT#4459

Reviewed-by: Rich Salz <rsalz@openssl.org>