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/memmove.c

@@ -0,0 +1,43 @@
+/*
+ *	memmove.c: memmove compat implementation.
+ *
+ *	Copyright (c) 2001-2008, NLnet Labs. All rights reserved.
+ *
+ * See LICENSE for the license.
+*/
+
+#include <ldns/config.h>
+#include <stdlib.h>
+
+void *memmove(void *dest, const void *src, size_t n);
+
+void *memmove(void *dest, const void *src, size_t n)
+{
+	uint8_t* from = (uint8_t*) src;
+	uint8_t* to = (uint8_t*) dest;
+
+	if (from == to || n == 0)
+		return dest;
+	if (to > from && to-from < (int)n) {
+		/* to overlaps with from */
+		/*  <from......>         */
+		/*         <to........>  */
+		/* copy in reverse, to avoid overwriting from */
+		int i;
+		for(i=n-1; i>=0; i--)
+			to[i] = from[i];
+		return dest;
+	}
+	if (from > to  && from-to < (int)n) {
+		/* to overlaps with from */
+		/*        <from......>   */
+		/*  <to........>         */
+		/* copy forwards, to avoid overwriting from */
+		size_t i;
+		for(i=0; i<n; i++)
+			to[i] = from[i];
+		return dest;
+	}
+	memcpy(dest, src, n);
+	return dest;
+}