+The adnsresfilter utility uses `tsearch' from the C library (a la SVID
+and X/Open). If your C library doesn't have tsearch you will find
+that configure arranges for adnsresfilter not to be built. To rectify
+this, install a C library containing tsearch, such as the GNU C
+library. It is best if tsearch uses an automatically-balancing tree
+algorithm, like the glibc version does. Simple binary trees may
+perform badly.
+
+You will probably find that GNU Make is required.