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/08-zonereader.tpkg/08-zonereader.dsc

@@ -0,0 +1,15 @@
+BaseName: 08-zonereader
+Version: 1.0
+Description: Check the backward compat. of the zone parser
+CreationDate: Mon Jan  2 16:24:38 CET 2006
+Maintainer: Jelte Jansen
+Category: 
+Component:
+Depends: 
+Help: 08-zonereader.help
+Pre: 
+Post:
+Test: 08-zonereader.test
+AuxFiles: 08-zonereader.outputzone 08-zonereader.inputzone
+Passed:
+Failure:

zonemaster-ldns/ldns/test/08-zonereader.tpkg/08-zonereader.help

@@ -0,0 +1,8 @@
+No arguments are used for this test.
+
+The example tool ldns-read-zone is used to read the zone from
+08-zonereader-inputzone that contains a number of different RRs. The output
+is compared to 08-zonereader-outputzone.
+
+With this test the zone reading functions are checked to see if they have
+not regressed.

zonemaster-ldns/ldns/test/08-zonereader.tpkg/08-zonereader.test

@@ -0,0 +1,19 @@
+# #-- 05-nm.test --#
+# source the master var file when it's there
+[ -f ../.tpkg.var.master ] && source ../.tpkg.var.master
+# use .tpkg.var.test for in test variable passing
+[ -f .tpkg.var.test ] && source .tpkg.var.test
+# svnserve resets the path, you may need to adjust it, like this:
+PATH=$PATH:/usr/sbin:/sbin:/usr/local/bin:/usr/local/sbin:.
+
+LIB=../../lib/
+export LD_LIBRARY_PATH=$LIB:$LD_LIBRARY_PATH
+
+# get the libdns symbols
+../../examples/ldns-read-zone -b 08-zonereader.inputzone > 08-zonereader.current
+
+diff 08-zonereader.current 08-zonereader.outputzone
+fail=$?
+rm -f 08-zonereader.current
+
+exit $fail