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/Changes

@@ -0,0 +1,53 @@
+Revision history for Perl extension DNS::LDNS.
+
+0.01  Thu Nov 22 12:48:29 2012
+	- original version; created by h2xs 1.23 with options
+		-A -n LDNS
+
+0.02  Fri Jan 18 09:47:57 2013
+        - Support for DNSSec and Resolver. Added some more constants from 
+	  the header files; created by h2xs 1.23 with options
+                -n LDNS /usr/include/ldns/ldns.h /usr/include/ldns/error.h /usr/include/ldns/rr.h /usr/include/ldns/keys.h /usr/include/ldns/packet.h /usr/include/ldns/resolver.h /usr/include/ldns/rdata.h /usr/include/ldns/dnssec.h
+
+0.03  Fri Apr 19 13:40:57 2013
+        - Renamed module to Net::LDNS
+
+0.04  Fri Dec 13 14:15:26 2013
+        - Renamed module to DNS::LDNS
+
+0.05  Mon Dec 30 10:14:00 2013
+        - Corrected versioning variable in all classes.
+        - Cleaned up the base class documentation.
+
+0.06  Tue Dec 31 12:17:00 2013
+        - Corrected pod syntax
+
+0.50  Sun Mar 30 11:05:23 2014
+        - Added prev parameter to the DNS::LDNS::RR::new(str) constructor.
+        - Corrected DNS::LDNS::RR::new(file/filename) constructor. Added prev
+          parameter, changed the default_ttl and origin parameters to 
+          references so they can return data back to the caller as intended.
+          Using the 'built-in' default values for ttl and origin, rather than
+          my own values.
+        - Corrected the DNS::LDNS::Zone::new() constructor. Corrected file
+          option for reading zone from stream. Using the 'built-in' default
+          values for ttl and origin, rather than my own values.
+        - Removed the $DNS::LDNS::DEFAULT_* variables, they proved to be less 
+          useful after modifying the Zone and RR constructors.
+        - More robust Makefile.PL. Check for existence of ldns library
+          and perl modules required for the test suite.
+
+0.51  Wed Apr 2 09:12:00 2014
+        - Added META.yml, and added some more package dependencies.
+        - Compatibility with ldns < 1.6.12.
+
+0.52  Tue May 5 09:13:00 2015
+        - Fixed typo in META.yml
+
+0.60  Thu Dec 29 11:15:00 2016
+        - Compatibility with ldns 1.7
+        - Dist::zilla build management
+
+0.61  Fri Dec 30 14:32:00 2016
+        - Bugfixes for ldns 1.7 compatibility
+        - Compatibility with perl 5.25