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/compat/isascii.c

@@ -0,0 +1,15 @@
+/* Just a replacement, if the original isascii is not
+   present */
+
+#if HAVE_CONFIG_H
+#include <ldns/config.h>
+#endif
+
+int isascii(int c);
+
+/* true if character is ascii. */
+int
+isascii(int c)
+{
+	return c >= 0 && c < 128;
+}