Win32 build broken by recent changes to xlog.c

Magnus Hagander wrote:

Seems it's dead on the buildfarm box as well:
http://www.pgbuildfarm.org/cgi-bin/show_log.pl?nm=snake&dt=2005-07-30%20
01:00:01

From what I can tell, the recent patch for O_DIRECT broke it.

OK, fix attached, and some macro cleanup too.

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