build: Improve test for ADNS
* configure.ac <adns>: Use adns_free as probe function for libadns. (HAVE_ADNS_FREE): Remove bogus tests to set this and remove the macro. (ADNSLIBS): Do not ac_subst - it is only used within configure.
adns_free is required on Windows anyway (for robustness reasons) and
it has been around for so long now that we do not need a separate
test. An upstream adns 1.5 has meanwhile been release but I doubt that
this has the required Windows code - and it is not libtool based
anyway.