Fixup check for unistd.h

Change-Id: I55411336d0ab81c2a9d33bcf854b400af9c857a9
Reviewed-on: http://review.couchbase.org/37582
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>