Fix build break on system without O_CLOEXEC

[stepinto]

On system without O_CLOEXEC, HAVE_O_CLOEXEC is defined as 0 in include/port/port_config.h, not undefined. Therefore, the right way to test it is "#if HAVE_O_CLOEXEC" rather than "#if defined(...)".

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[hebasto]

Also see https://github.com/google/leveldb/pull/1028.