Build IOCP as an OBJECT library

This lets us generate a static archive with mingw that can be linked
against properly

Change-Id: Ia7cc7c90a993a1682bb223aab2f7542d3c79c138
Reviewed-on: http://review.couchbase.org/43697
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>