In a gps receiver, how is the delay of the signal in the antenna lead allowed for?

Typically they have short or no leads - but note that a uniform extra delay through the system isn't a problem, it just means that the GPS tells you where the antenna is not where the decoder is.

This is magnified in situations where GPS is re-broadcast indoors. Again, it gives you the location of the external antenna not the decoder.

There is usually no need for any compensation at all. The GPS receiver measures the location of the phase center of the antenna. What happens between the phase center of the antenna and the receiver affects all of the satellite signals equally and hence will not affect the position calculation. If a precise time is needed, then the propagation delay of the cable is compensated for in the timing outputs of the receiver.