Check for Starlink on $STARLINK_DIR being set

We can not actually trust $STARLINK because the configure script
will force $STARLINK to be /star which messes the test up even
if /star does not actually exist. STARLINK_DIR is not twiddled
by configure so is a safe choice and means that we do not
need to panic if --without-starlink has been forgotten on the
configure line.
Add note about --without-starlink to README

Although that may play badly now that STARLINK_DIR is being used
instead of $STARLINK.
V0.5.1