ORIGINAL: Shoe
My question was: In both cases, the glider's potential energy changed by the same amount. In the no-wind case, the glider's kinetic energy didn't change at all (in the earth-fixed reference frame). In the headwind case, the glider's kinteic changed by a lot (also in the earth-fixed reference frame). Where did all that extra kinetic energy come from? It had to come from somewhere.
It comes from relative velocity, by definition. It only has a direct effect on the airplane when the airplane is in contact with the earth. Otherwise you would have to consider it's motion relative to the sun, moon, and other planets as well.