Add missing macros to support Arduino168. Ref code by rocketscream

[YorkshireKev]

Hi,

I have applied the conditional macro definition to support Arduino168. Successfully tested with generic Arduino168 clone - using 8 second sleep timer and noted significant reduced power consumption while device is sleeping (multimeter milliamp test).

I would like this merged because there are loads of really cheap 168 clones available that make great battery powered projects. These would surely benefit from this excellent library.

Thanks, Kev.