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/process.h

@@ -0,0 +1,51 @@
+/*
+ * process.h, process queries to determine the answer RRs and packets.
+*/
+
+#ifndef PROCESS_H
+#define PROCESS_H
+
+struct socket_service;
+struct zones_t;
+struct zone_entry_t;
+
+/**
+ * Calculate the reply for the query in q. Put answer in packet p.
+ * P is an empty packet on routine start, with header.
+ */
+ldns_pkt_rcode process_pkts(struct socket_service* sv, 
+	ldns_pkt* q, ldns_pkt* r, struct zones_t* zones);
+
+/**
+ * Process QUERY
+ */
+ldns_pkt_rcode process_pkt_query(struct socket_service* sv, 
+	ldns_pkt* q, ldns_pkt* r, struct zones_t* zones);
+
+/**
+ * Process NOTIFY
+ */
+ldns_pkt_rcode process_pkt_notify(struct socket_service* sv, 
+	ldns_pkt* q, ldns_pkt* r, struct zones_t* zones);
+
+/**
+ * Process SOA queries
+ * pass correct zone entry.
+ */
+ldns_pkt_rcode process_pkt_soa(struct socket_service* sv, 
+	ldns_pkt* q, ldns_pkt* r, struct zone_entry_t* entry);
+
+/**
+ * Process IXFR
+ * pass correct zone entry.
+ */
+ldns_pkt_rcode process_pkt_ixfr(struct socket_service* sv, 
+	ldns_pkt* q, ldns_pkt* r, struct zone_entry_t* entry);
+
+/**
+ * Process AXFR
+ * pass correct zone entry.
+ */
+ldns_pkt_rcode process_pkt_axfr(struct socket_service* sv, 
+	struct zone_entry_t* entry);
+#endif /* PROCESS_H */