Enable ldns when using ldns-config.

Actually enable ldns when attempting to use ldns-config.  bz#2697, patch
from fredrik at fornwall.net.

configure.ac

@@ -1486,6 +1486,7 @@ AC_ARG_WITH(ldns,
 		else
 			LIBS="$LIBS `$LDNSCONFIG --libs`"
 			CPPFLAGS="$CPPFLAGS `$LDNSCONFIG --cflags`"
+			ldns=yes
 		fi
 	elif test "x$withval" != "xno" ; then
 			CPPFLAGS="$CPPFLAGS -I${withval}/include"