The changes are non-functional changes for normal InnoDB,
but needed for building the Hot Backup on Windows (with
UNIV_HOTBACKUP defined).

- Define os_aio_use_native_aio for HB.
- Do not acquire seek mutexes for backup since HB is single threaded.
- Do not use srv_flush_log_at_trx_commit for HB build

rb://155

Approved by: Marko