Support PostgreSQL 19

[wolfgangwalther]

Currently fails to build with PostgreSQL 19 beta 1. Tail of the log:

/nix/store/g6543ym63fizn39ibhkfi87z3bhxvghx-postgresql-19beta1-dev/include/server/varatt.h:361:39: note: passing argument to parameter 'PTR' here
  361 | VARATT_IS_EXTERNAL_ONDISK(const void *PTR)
      |                                       ^
wal2json.c:2141:68: error: incompatible integer to pointer conversion passing 'Datum' (aka 'unsigned long') to parameter of type 'const void *' [-Wint-conversion]
 2141 |                 if (!nulls[i] && attr->attlen == -1 && VARATT_IS_EXTERNAL_ONDISK(values[i]))
      |                                                                                  ^~~~~~~~~
/nix/store/g6543ym63fizn39ibhkfi87z3bhxvghx-postgresql-19beta1-dev/include/server/varatt.h:361:39: note: passing argument to parameter 'PTR' here
  361 | VARATT_IS_EXTERNAL_ONDISK(const void *PTR)
      |                                       ^
wal2json.c:2381:79: error: no member named 'xact_time' in 'struct ReorderBufferTXN'
 2381 |                 appendStringInfo(ctx->out, ",\"timestamp\":\"%s\"", timestamptz_to_str(txn->xact_time.commit_time));
      |                                                                                        ~~~  ^
wal2json.c:2736:80: error: no member named 'xact_time' in 'struct ReorderBufferTXN'
 2736 |                         appendStringInfo(ctx->out, ",\"timestamp\":\"%s\"", timestamptz_to_str(txn->xact_time.commit_time));
      |                                                                                                ~~~  ^
wal2json.c:2965:80: error: no member named 'xact_time' in 'struct ReorderBufferTXN'
 2965 |                         appendStringInfo(ctx->out, ",\"timestamp\":\"%s\"", timestamptz_to_str(txn->xact_time.commit_time));
      |                                                                                                ~~~  ^
16 errors generated.
make: *** [<builtin>: wal2json.o] Error 1

[eulerto]

If you build from master it works fine. I'm planning a new release before Postgres 19 RC1.