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/python/docs/source/install.rst

@@ -0,0 +1,72 @@
+Installation
+===================================
+
+**Prerequisites**
+
+SWIG 1.3 and GNU make are required to build modules for Python 2.4 and higher
+(but lower than 3). In order to build modules for Python 3.2 or higher,
+SWIG in version 2.0.4 or higher is required.
+
+Note that Python 3.0 and 3.1 are not supported.
+
+In order to build this documentation the Sphinx Python documentation generator
+is required.
+
+**Download**
+
+The latest source codes can be downloaded from `here`_.
+
+.. _here: http://nlnetlabs.nl/projects/ldns/
+
+**Compiling**
+
+After downloading the source code archive (this example uses
+ldns-1.6.13.tar.gz), pyLDNS can be enabled and compiled by typing::
+
+	> tar -xzf ldns-1.6.13.tar.gz
+	> cd ldns-1.6.13
+	> ./configure --with-pyldns
+	> make
+
+You need GNU make to compile pyLDNS; SWIG and Python development libraries to
+compile the extension module.
+
+**Selecting Target Python Interpreter**
+
+By default, the pyLDNS module builds for the default Python interpreter (i.e.,
+the Python interpreter which can be accessed by just typing ``python`` in
+the command line). If you desire to build the pyLDNS module for a different
+Python version then you must specify the desired Python version by setting
+the ``PYTHON_VERSION`` variable during the configure phase::
+
+	> PYTHON_VERSION=3.2 ./configure --with-pyldns
+	> make
+
+By default the pyLDNS compiles from sources for a single Python interpreter.
+Remember to execute scripts requiring pyLDNS in those Python interpreters which
+have pyLDNS installed.
+
+**Testing**
+
+If the compilation is successful, you can test the python LDNS extension module
+by executing the commands::
+
+	> cd contrib/python
+	> make testenv
+	> ./ldns-mx.py
+
+Again, remember to use the Python interpreter version which the pyLDNS module
+has been compiled with.
+
+The commands will start a new shell, in which several symbolic links will be
+set-up. When you exit the shell, then symbolic links will be deleted.
+
+In ``contrib/python/examples`` several simple Python scripts utilising pyLDNS
+can be found. These scripts demonstrate the capabilities of the LDNS library.
+
+**Installation**
+
+To install the libraries and it's extensions type::
+
+	> cd ldns-1.6.13
+	> make install