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/compat/calloc.c

@@ -0,0 +1,24 @@
+/* Just a replacement, if the original malloc is not
+   GNU-compliant. See autoconf documentation. */
+
+#if HAVE_CONFIG_H
+#include <ldns/config.h>
+#endif
+
+void *calloc();
+
+#if !HAVE_BZERO && HAVE_MEMSET
+# define bzero(buf, bytes)	((void) memset (buf, 0, bytes))
+#endif
+
+void *
+calloc(size_t num, size_t size)
+{
+	void *new = malloc(num * size);
+	if (!new) {
+		return NULL;
+	}
+	bzero(new, num * size);
+	return new;
+}
+