Since the quad extends from [x, y] to [x+w, y+h], the center is located at [x+w/2, y+h/2]. So first translate the center to the origin, rotate, and translate back. Something like this.
glTranslate(x+w/2, y+h/2, 0);glRotate(whatever);glTranslate(-x-w/2, -y-h/2, 0);