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/zones.h

@@ -0,0 +1,76 @@
+/*
+	zones.h		keep list of zones to handle with backend info.
+*/
+
+#ifndef ZONES_H
+#define ZONES_H
+struct zinfo_t;
+
+/**
+ * zone entry information
+ */
+struct zone_entry_t {
+	/** rbtree node info */
+	ldns_rbnode_t node;
+	/** zone name */
+	ldns_rdf* zname;
+	/** zone name in text */
+	char* zstr;
+	/** zone class */
+	uint16_t zclass;
+	/** zone info */
+	struct zinfo_t* zinfo;
+	
+	/** next in linked list */
+	struct zone_entry_t* next;
+};
+
+/**
+ * Keep zones information.
+*/
+struct zones_t {
+	/** tree of zone_entry, sorted by zname, zclass */
+	ldns_rbtree_t* ztree;
+};
+
+/**
+ * create new empty zones struct
+*/
+struct zones_t* zones_create(void);
+
+/**
+ * Read all the zone entry storage
+ */
+void zones_read(struct zones_t* zones);
+
+/**
+ * Find zone entry given the name
+ * or NULL if doesn't exist
+*/
+struct zone_entry_t* zones_find(struct zones_t* zones, const char* name,
+	uint16_t fclass);
+
+/**
+ * find a zone based on rdf 
+ */
+struct zone_entry_t* zones_find_rdf(struct zones_t* zones, ldns_rdf* name,
+	uint16_t fclass);
+
+/**
+ * Insert new entry for zone name, returns new entry.
+ * or NULL if out of memory.
+*/
+struct zone_entry_t* zones_insert(struct zones_t* zones, const char* name,
+	uint16_t nclass);
+
+/**
+ * free a zone entry
+ */
+void zone_entry_free(struct zone_entry_t* entry);
+
+/**
+ * free zones data structure
+*/
+void zones_free(struct zones_t* zones);
+
+#endif /* ZONES_H */