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/realloc.c

@@ -0,0 +1,30 @@
+/* Just a replacement, if the original malloc is not
+   GNU-compliant. Based on malloc.c */
+
+#if HAVE_CONFIG_H
+#include <ldns/config.h>
+#endif
+#undef realloc
+
+#include <sys/types.h>
+
+void *realloc (void*, size_t);
+void *malloc (size_t);
+
+/* Changes allocation to new sizes, copies over old data.
+ * if oldptr is NULL, does a malloc.
+ * if size is zero, allocate 1-byte block....
+ *   (does not return NULL and free block)
+ */
+
+void *
+rpl_realloc (void* ptr, size_t n)
+{
+  if (n == 0)
+    n = 1;
+  if(ptr == 0) {
+    return malloc(n);
+  }
+  return realloc(ptr, n);
+}
+