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/m4/ax_have_poll.m4

@@ -0,0 +1,72 @@
+# ===========================================================================
+#       https://www.gnu.org/software/autoconf-archive/ax_have_poll.html
+# ===========================================================================
+#
+# SYNOPSIS
+#
+#   AX_HAVE_POLL([ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND])
+#   AX_HAVE_PPOLL([ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND])
+#
+# DESCRIPTION
+#
+#   This macro determines whether the system supports the poll I/O event
+#   interface. A neat usage example would be:
+#
+#     AX_HAVE_POLL(
+#       [AX_CONFIG_FEATURE_ENABLE(poll)],
+#       [AX_CONFIG_FEATURE_DISABLE(poll)])
+#     AX_CONFIG_FEATURE(
+#       [poll], [This platform supports poll(7)],
+#       [HAVE_POLL], [This platform supports poll(7).])
+#
+#   Some systems -- most notably Linux kernel 2.6.16 and later -- also have
+#   the variant ppoll(). The availability of that function can be tested
+#   with the second macro. Generally speaking, it is safe to assume that
+#   AX_HAVE_POLL would succeed if AX_HAVE_PPOLL has, but not the other way
+#   round.
+#
+# LICENSE
+#
+#   Copyright (c) 2009 Peter Simons <simons@cryp.to>
+#
+#   Copying and distribution of this file, with or without modification, are
+#   permitted in any medium without royalty provided the copyright notice
+#   and this notice are preserved. This file is offered as-is, without any
+#   warranty.
+
+#serial 8
+
+AC_DEFUN([AX_HAVE_POLL], [dnl
+  AC_MSG_CHECKING([for poll(2)])
+  AC_CACHE_VAL([ax_cv_have_poll], [dnl
+    AC_LINK_IFELSE([dnl
+      AC_LANG_PROGRAM(
+        [#include <poll.h>],
+        [int rc; rc = poll((struct pollfd *)(0), 0, 0);])],
+      [ax_cv_have_poll=yes],
+      [ax_cv_have_poll=no])])
+  AS_IF([test "${ax_cv_have_poll}" = "yes"],
+    [AC_MSG_RESULT([yes])
+$1],[AC_MSG_RESULT([no])
+$2])
+])dnl
+
+AC_DEFUN([AX_HAVE_PPOLL], [dnl
+  AC_MSG_CHECKING([for ppoll(2)])
+  AC_CACHE_VAL([ax_cv_have_ppoll], [dnl
+    AC_LINK_IFELSE([dnl
+      AC_LANG_PROGRAM(
+        [dnl
+#include <poll.h>
+#include <signal.h>],
+        [dnl
+int rc;
+rc = poll((struct pollfd *)(0), 0, 0);
+rc = ppoll((struct pollfd *)(0), 0, (struct timespec const *)(0), (sigset_t const *)(0));])],
+      [ax_cv_have_ppoll=yes],
+      [ax_cv_have_ppoll=no])])
+  AS_IF([test "${ax_cv_have_ppoll}" = "yes"],
+    [AC_MSG_RESULT([yes])
+$1],[AC_MSG_RESULT([no])
+$2])
+])