This would depend on the wording of the original contract. If the agreement is for the buyer of the land to build a fence around the farmhouse, then it can be enforced, but if the agreement is between the two individuals (farmer and buyer) and not stipulated specifically to the land/buildings, then it's not going to be something too easy to enforce.

My instincts are that it would be enforceable, because the latter's a strange way of doing things and the contract should name the farmhouse as the building to be fenced off. Therefore, the buyer of the land can be forced to build the fence.

If there's been a secondary contract/agreement not to bother with the fence (or the fence was put up and then taken down) then that's something you would need to investigate. If, however, the only paperwork is the contract stating that Johnny LandBuyer has to build a fence around the farm, then you should be fully able to enforce it.