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/testbed/unitmain.c

@@ -0,0 +1,31 @@
+/*
+ * testcode/unitmain.c - unit test main program for unbound.
+ *
+ * Copyright (c) 2007, NLnet Labs. All rights reserved.
+ *
+ * See LICENSE for the license.
+ *
+ */
+/**
+ * \file
+ * Unit test main program. Calls all the other unit tests.
+ * Exits with code 1 on a failure. 0 if all unit tests are successful.
+ */
+
+#include "config.h"
+
+/**
+ * Main unit test program. Setup, teardown and report errors.
+ * @param argc: arg count.
+ * @param argv: array of command line arguments.
+ */
+int main(int argc, char* argv[])
+{
+	if(argc != 1) {
+		printf("usage: %s\n", argv[0]);
+		printf("\tperforms unit tests.\n");
+		return 1;
+	}
+	printf("Start of %s unit test.\n", PACKAGE_STRING);
+	return 0;
+}