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/examples/ldns-dpa.h

@@ -0,0 +1,92 @@
+#include <stdio.h>
+#include <string.h>
+#include <unistd.h>
+#include <assert.h>
+
+#if STDC_HEADERS
+#include <stdlib.h>
+#include <stddef.h>
+#endif
+
+#ifdef HAVE_STDINT_H
+#include <stdint.h>
+#endif
+
+#ifdef HAVE_SYS_SOCKET_H
+#include <sys/socket.h>
+#endif
+
+#ifdef HAVE_NETINET_IN_H
+#include <netinet/in.h>
+#endif
+
+#ifdef HAVE_ARPA_INET_H
+#include <arpa/inet.h>
+#endif
+
+#ifdef HAVE_NETINET_UDP_H
+#include <netinet/udp.h>
+#endif
+
+#ifdef HAVE_TIME_H
+#include <time.h>
+#endif
+
+#ifdef HAVE_PCAP_H
+#include <pcap.h>
+#endif
+
+#ifdef HAVE_NETINET_IN_SYSTM_H
+#include <netinet/in_systm.h>
+#endif
+
+#ifdef HAVE_NETINET_IP_H
+#include <netinet/ip.h>
+#endif
+
+#ifdef HAVE_NET_IF_H
+#include <net/if.h>
+#endif
+
+#ifdef HAVE_NETINET_IF_ETHER_H
+#include <netinet/if_ether.h>
+#endif
+
+#ifdef HAVE_WINSOCK2_H
+#define USE_WINSOCK 1
+#include <winsock2.h>
+#endif
+
+#ifdef HAVE_WS2TCPIP_H
+#include <ws2tcpip.h>
+#endif
+
+#ifndef HAVE_GETADDRINFO
+#include <fake-rfc2553.h>
+#endif
+
+#ifndef HAVE_RANDOM
+/* random can be replaced by rand for ldnsexamples */
+#define random rand
+#endif
+
+#ifndef HAVE_SRANDOM
+/* srandom can be replaced by srand for ldnsexamples */
+#define srandom srand
+#endif
+
+extern char *optarg;
+extern int optind, opterr;
+
+#ifndef EXIT_FAILURE
+#define EXIT_FAILURE  1
+#endif
+#ifndef EXIT_SUCCESS
+#define EXIT_SUCCESS  0
+#endif
+
+#ifdef S_SPLINT_S
+#define FD_ZERO(a) /* a */
+#define FD_SET(a,b) /* a, b */
+#endif
+