I think you could define a supercar as simply:
2 seats (Maclaren F1 excepted).
Engine to the middle or rear of car.
Very expensive.
Very fast.
Radical looking.
Low, wide, impractical, and noisy.
Use of advanced and/or motor race technology.
Judging by this criteria, the Audi R8, Honda NSX, Ford GT40 are all supercars. The Ferrari 550, Jaguar XK8, Aston Martin DB9 etc are classed as GT cars.
For anyone saying that Supercars can’t be made by mainstream manufacturers let me just say that Ferrari is owned and controlled by Fiat, and Lamborghini and Bugatti are owned by the VAG group. Honda also has a fantastic pedigree in motorsport and F1, as does Ford, as does Audi.