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/test/14-read-zone.tpkg/14-read-zone.dsc

@@ -0,0 +1,15 @@
+BaseName: 14-read-zone
+Version: 1.0
+Description: read a zone with comment on the last line
+CreationDate: Thu Apr  6 10:50:19 CEST 2006
+Maintainer: Miek Gieben
+Category: 
+Component:
+Depends: 
+Help: 14-read-zone.help
+Pre: 
+Post: 
+Test: 14-read-zone.test
+AuxFiles: 14-read-zone.db
+Passed:
+Failure: