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/pcat/getdelim.c

@@ -0,0 +1,63 @@
+#include "config.h"
+#ifndef HAVE_GETDELIM
+
+#define GETDELIM_BUFFER 128
+
+/* copied from xine-devel */
+size_t
+getdelim( char **lineptr, size_t *n, int delimiter, FILE *stream )
+{
+        char *p;
+        int c;
+        size_t len = 0;
+
+        if (!lineptr || !n || (!*lineptr && *n))
+                return -1;
+
+        /* allocate initial buffer */
+        if (!*lineptr || !*n) {
+                char *np;
+                np = realloc( *lineptr, GETDELIM_BUFFER );
+                if (!np)
+                        return -1;
+                *n = GETDELIM_BUFFER;
+                *lineptr = np;
+        }
+
+        p = *lineptr;
+
+        /* read characters from stream */
+        while ((c = fgetc( stream )) != EOF) {
+                if (len >= *n) {
+                        char *np = realloc( *lineptr, *n * 2 );
+                        if (!np)
+                                return -1;
+                        p = np + (p - *lineptr);
+                        *lineptr = np;
+                        *n *= 2;
+                }
+                *p++ = (char) c;
+                len++;
+                if (delimiter == c)
+                        break;
+        }
+
+        /* end of file without any bytes read */
+        if ((c == EOF) && (len == 0))
+                return -1;
+
+        /* trailing "\0" */
+        if (len >= *n) {
+                char *np = realloc( *lineptr, *n + 1 );
+                if (!np)
+                        return -1;
+                p = np + (p - *lineptr);
+                *lineptr = np;
+                *n += 1;
+        }
+        *p = '\0';
+
+        return len;
+}
+
+#endif /* !HAVE_GETDELIM */