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/README

@@ -0,0 +1,108 @@
+
+Contents: 
+	REQUIREMENTS
+	INSTALLATION
+		libdns
+		examples
+		drill
+	INFORMATION FOR SPECIFIC OPERATING SYSTEMS
+		Mac OS X
+		Solaris
+	KNOWN ISSUES
+		pyldns
+        Your Support
+
+Project page:
+http://www.nlnetlabs.nl/ldns/
+On that page you can also subscribe to the ldns mailing list.
+
+* Development 
+ldns is mainly developed on Linux and FreeBSD. It is regularly tested to
+compile on other systems like Solaris and Mac OS X.
+
+REQUIREMENTS
+- OpenSSL (Optional, but needed for features like DNSSEC)
+  - OpenSSL >= 0.9.7f for DANE support
+  - OpenSSL >= 1.0.0  for ECDSA and GOST support
+- libpcap (Optional, but needed for examples/ldns-dpa)
+- (GNU) libtool (in OSX, that's glibtool, not libtool)
+- GNU make
+
+INSTALLATION
+1. Unpack the tarball
+2. cd ldns-<VERSION>
+3. ./configure --with-examples --with-drill
+   (optionally compile python bindings too with: --with-pyldns)
+4. make
+5. make install
+
+
+* Building from repository
+
+If you are building from the repository you will need to have (gnu)
+autotools like libtool and autoreconf installed. A list of all the commands
+needed to build everything can be found in README.git. Note that the actual
+commands may be a little bit different on your machine. Most notably, you'll
+need to run libtoolize (or glibtoolize). If you skip this step, you'll get
+an error about missing config.sub.
+
+* Developers
+ldns is developed by the ldns team at NLnet Labs. This team currently
+consists of:
+  o Willem Toorop
+  o Wouter Wijngaards
+
+Former main developers:
+  o Jelte Jansen
+  o Miek Gieben
+  o Matthijs Mekking
+
+* Credits
+We have received patches from the following people, thanks!
+  o Bedrich Kosata
+  o Erik Rozendaal
+  o Håkan Olsson
+  o Jakob Schlyter
+  o Paul Wouters
+  o Simon Vallet
+  o Ondřej Surý
+  o Karel Slany
+  o Havard Eidnes
+  o Leo Baltus
+  o Dag-Erling Smørgrav
+  o Felipe Gasper
+
+
+INFORMATION FOR SPECIFIC OPERATING SYSTEMS
+
+MAC OS X
+
+For MACOSX 10.4 and later, it seems that you have to set the
+MACOSX_DEPLOYMENT_TARGET environment variable to 10.4 before running
+make. Apparently it defaults to 10.1.
+
+This appears to be a known problem in 10.2 to 10.4, see:
+http://developer.apple.com/qa/qa2001/qa1233.html
+for more information.
+
+
+SOLARIS
+
+In Solaris multi-architecture systems (which have both 32-bit and
+64-bit support), it can be a bit taxing to convince the system to
+compile in 64-bit mode. Jakob Schlyter has kindly contributed a build
+script that sets the right build and link options. You can find it in
+contrib/build-solaris.sh
+
+KNOWN ISSUES
+
+A complete list of currently known open issues can be found here:
+https://github.com/NLnetLabs/ldns/issues
+
+* pyldns
+Compiling pyldns produces many ``unused parameter'' warnings.  Those are
+harmless and may safely be ignored.
+Also, when building with SWIG older than 2.0.4, compiling
+pyldns produces many ``missing initializer'' warnings. Those are harmless
+too.
+