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/lua/configure.ac

@@ -0,0 +1,83 @@
+#                                               -*- Autoconf -*-
+# Process this file with autoconf to produce a configure script.
+
+AC_PREREQ(2.57)
+AC_INIT(ldns-tests, 1.7.0, dns-team@nlnetlabs.nl, ldns-tests-1.0)
+AC_CONFIG_SRCDIR([../ldns/config.h])
+
+AC_AIX
+# Checks for programs.
+AC_PROG_CC
+AC_PROG_MAKE_SET
+
+# Checks for libraries.
+# Checks for header files.
+#AC_HEADER_STDC
+#AC_HEADER_SYS_WAIT
+# do the very minimum - we can always extend this
+AC_CHECK_HEADERS([getopt.h stdlib.h stdio.h assert.h netinet/in.hctype.h])
+AC_CHECK_HEADERS(sys/param.h sys/mount.h,,,
+[
+  [
+   #if HAVE_SYS_PARAM_H
+   # include <sys/param.h>
+   #endif
+  ]
+])
+
+# ripped from http://autoconf-archive.cryp.to/check_ssl.html
+
+
+# check for ldns
+AC_ARG_WITH(ldns, AC_HELP_STRING([--with-ldns=pathname],[]))
+AC_MSG_CHECKING(for ldns/ldns.h)
+for dir in $withval /usr/local/ldns /usr/lib/ldns /usr/ldns /usr/pkg /usr/local /usr; do
+ldnsdir="$dir"
+if test -f "$dir/include/ldns/ldns.h"; then
+    found_ldns="yes";
+    CFLAGS="$CFLAGS -I$ldnsdir/include/ -DHAVE_LDNS";
+    CXXFLAGS="$CXXFLAGS -I$ldnsdir/include/ -DHAVE_LDNS";
+    break;
+fi
+if test -f "$dir/ldns/ldns.h"; then
+    found_ldns="yes";
+    CFLAGS="$CFLAGS -I$ldnsdir/ -DHAVE_LDNS";
+    CXXFLAGS="$CXXFLAGS -I$ldnsdir/ -DHAVE_LDNS";
+    break
+fi
+done
+if test x_$found_ldns != x_yes; then
+        AC_MSG_RESULT(no)
+	AC_MSG_ERROR(Cannot find ldns libraries)
+else
+#        printf "ldns found in $ldnsdir\n";
+	LIBS="$LIBS -lldns";
+	LDFLAGS="$LDFLAGS -L$ldnsdir/lib";
+	LDFLAGS="$LDFLAGS -L$ldnsdir/.libs"; # hack for dev.
+	HAVE_LDNS=yes
+        AC_MSG_RESULT(yes)
+fi
+AC_SUBST(HAVE_LDNS)
+
+# I don't use these
+# Checks for typedefs, structures, and compiler characteristics.
+#AC_TYPE_UID_T
+#AC_TYPE_MODE_T
+#AC_TYPE_OFF_T
+#AC_TYPE_SIZE_T
+#AC_STRUCT_TM
+
+# Checks for library functions.
+# check for ldns
+#AC_FUNC_CHOWN
+#AC_FUNC_FORK
+#AC_FUNC_MALLOC
+#AC_FUNC_MKTIME
+#AC_FUNC_STAT
+#AC_CHECK_FUNCS([mkdir rmdir strchr strrchr strstr])
+
+#AC_DEFINE_UNQUOTED(SYSCONFDIR, "$sysconfdir")
+
+AC_CONFIG_FILES([Makefile])
+AC_CONFIG_HEADER([config.h])
+AC_OUTPUT