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/f

@@ -0,0 +1,13 @@
+#!/usr/bin/env bash
+
+# get all the functions from ldns
+
+FILES="buffer.h error.h host2str.h keys.h packet.h rdata.h rr_functions.h str2host.h update.h wire2host.h
+common.h dname.h dnssec.h higher.h host2wire.h net.h parse.h resolver.h
+rr.h tsig.h util.h zone.h"
+
+# we need two grep ldns_ ...
+for i in $FILES; do 
+        cpp -I.. "../ldns/$i"  | grep -E '.*? .*?\(.*' | grep ldns_  | \
+        awk '{ print $2 }' | sed 's/^\*//' | sed 's/(.*$//' | grep ldns_
+done