fix: populate ldns submodule and add autotools to LDNS build stage

- Re-cloned zonemaster-ldns with --recurse-submodules so the bundled
  ldns C library source (including Changelog and configure.ac) is present
- Added autoconf, automake, libtool to Dockerfile.backend ldns-build stage
  so libtoolize + autoreconf can generate ldns/configure during make

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

zonemaster-ldns/ldns/test/tpkg/examples/test3.tpkg/test3.test

@@ -0,0 +1,10 @@
+#!/bin/sh
+
+dig miek.nl | grep AAAA | sed 's/[ \t]*//g' > $$.digout
+diff -u test3.querygood $$.digout
+if [ $? -ne 0 ]; then 
+        echo "Query failed"
+else 
+        echo "Query matched"
+fi
+rm -f $$.digout