Define WINVER and _WIN32_WINNT as 0x6000

This fixes some builds where WINVER is by default 0x0500 (XP) and does
not have OVERLAPPED_ENTRY

Change-Id: If4cec2c1a56407ecacb030cd34c7d64fb510ef89
Reviewed-on: http://review.couchbase.org/33323
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>