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/masterdont/config.h

@@ -0,0 +1,26 @@
+
+/* standard includes */
+#include <stdio.h>
+#include <string.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <stdint.h>
+#include <getopt.h>
+#include <errno.h>
+#include <signal.h>
+
+/* for networking */
+#include <sys/time.h>
+#include <sys/types.h>
+#include <sys/socket.h>
+#include <netdb.h>
+#include <arpa/inet.h>
+#include <sys/select.h>
+#include <fcntl.h>
+#include <sys/stat.h>
+
+/* ldns */
+#include "ldns/ldns.h"
+
+#define DEFAULT_CONFIG  "example/masterdont.conf"
+