This might sound like a stupid question to ask but I want to know if there's another approach to countering builder harass while I'm constructing a building. Sure I can get another SCV to back him up but I don't want to waste two SCVs, one for scouting and another one chasing the scout.
You have to use another SCV.
That you do is factored into the balance of the game. For gosh sakes man, you have mules, it's enough money.
Edit: I believe Mark's answer is wrong. There's no way to prevent a scouting worker from seeing your build with a SCV and that has nothing to with whether or not you should use a second SCV.
It's not really a waste if you prevent valuable information from getting to you opponent. For instance seeing what your build is. Also you reduce the risk of a protoss player from setting up a proxy or cannon rush/containment.
Also as Terran you run the risk of them attacking your scv as it's building. you should chase him out with a second scv until you have an army unit out.