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-resolver.c

@@ -0,0 +1,47 @@
+/*
+ * ldns-resolver tries to create a resolver structure from /dev/urandom
+ * this is only useful to test the library for robustness with input data
+ *
+ * (c) NLnet Labs 2006 - 2008
+ * See the file LICENSE for the license
+ */
+
+#include "config.h"
+#include "errno.h"
+
+#include <ldns/ldns.h>
+
+int
+main(int argc, char **argv) {
+
+	ldns_resolver *r;
+	int line = 1;
+	FILE *rand;
+	ldns_status s;
+
+	if (argc != 2 || strncmp(argv[1], "-h", 3) == 0) {
+		printf("Usage: ldns-resolver <file>\n");
+		printf("Tries to create a stub resolver structure from the given file.\n");
+		exit(EXIT_FAILURE);
+	}
+	
+	if (!(rand = fopen(argv[1], "r"))) {
+		printf("Error opening %s: %s\n", argv[1], strerror(errno));
+		exit(EXIT_FAILURE);
+	}
+
+	printf("Trying to read from %s\n", argv[1]);
+	s = ldns_resolver_new_frm_fp_l(&r, rand, &line);
+	if (s != LDNS_STATUS_OK) {
+		printf("Failed: %s at line %d\n", ldns_get_errorstr_by_id(s), line);
+		exit(EXIT_FAILURE);
+	} else {
+		printf("Success\n");
+		ldns_resolver_print(stdout, r);
+		ldns_resolver_deep_free(r);
+	}
+
+	fclose(rand);
+
+	return EXIT_SUCCESS;
+}