Make tree compilable (+WAL).

src/backend/access/transam/xact.c

src/backend/access/transam/xlog.c

src/backend/commands/vacuum.c

src/backend/storage/buffer/bufmgr.c

src/backend/storage/smgr/md.c

src/backend/utils/init/postinit.c

src/include/storage/bufmgr.h