Fix Windows compilation for AMD64 (64-bits)

[AldoMX]

Historical Context

I remember attempting to compile the game for 64 bits, and I remember that such attempt failed.

I don't know how big will be the effort to support 64-bits. I don't know if it will affect other dependencies like ffmpeg which requires a specific version. It may probably be a big effort and it will better to abandon the idea of supporting 64-bits.

What needs to be done?

Attempt to compile the project for AMD64.

Update

The effort looks indeed big, but the SM team managed to do it: stepmania#1899