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/contrib/DNS-LDNS/dist.ini

@@ -0,0 +1,18 @@
+name    = DNS-LDNS
+author  = Erik Ostlyngen <erik@uninett.no>
+copyright_holder = UNINETT Norid AS
+copyright_year = 2013
+license = None
+version = 0.61
+
+[AutoPrereqs]
+[GatherDir]
+[MetaYAML]
+[MakeMaker::Awesome]
+WriteMakefile_arg = LIBS => ['-lldns']
+footer_file = constants.PL
+[CheckLib]
+lib = ldns
+header = ldns/ldns.h
+[License]
+[PruneCruft]