Do I have to pay taxes on the gain from the sale of land if I put the gain towards the cost of building a second home if the second home is not finished within a year?

There’s no tax break for investing the proceeds in another asset. 

yes the gain on the sale of land is taxable.

Q. Do I have to pay taxes on the gain from the sale of land if I put the gain towards the cost of building a second home if the second home is not finished within a year?

A. Yes. You may be thinking about an old rule that expired around 1998.  Even back then, your situation would not have qualified for the tax break.