--with-ldns flag is obsolete

[orlitzky]

The --with-ldns flag is supposedly for use with --with-unbound, but Unbound itself hasn't used ldns in a decade. It looks like the flag was only originally needed to fix libunbound underlinking or something like that; no actual code gets enabled in OpenDKIM, it only adds -lldns to the list of libraries to link.

Tests pass without it (and with Unbound).