Use _WIN32 instead of WIN32

_WIN32 is a predefined symbol from the MSVC

Change-Id: Ibc36ef706a189ce07ada9ae4be3426e87f5c7496
Reviewed-on: http://review.couchbase.org/25160
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>