cmake/configure: Force unix makefiles

On OS X cmake generates an Xcode project by default

Change-Id: I89a0692a91a288c6d015539fb7968d9a689c3af8
Reviewed-on: http://review.couchbase.org/33853
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>