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/13-unit-tests-base.tpkg/13-unit-tests-base.dsc

@@ -0,0 +1,16 @@
+BaseName: 13-unit-tests-base
+Version: 1.0
+Description: Run unit tests on base 32 / 64 converters and sha1/sha2 functions
+CreationDate: Wed Mar 15 10:15:57 CET 2006
+Maintainer: Jelte Jansen
+Category: 
+Component:
+CmdDepends: 
+Depends: 
+Help: 13-unit-tests-base.help
+Pre: 13-unit-tests-base.pre
+Post: 
+Test: 13-unit-tests-base.test
+AuxFiles: 13-unit-tests-base.Makefile.in 13-unit-tests-base.configure.ac 13-unit-tests-base.c
+Passed:
+Failure: