No need for this guard; who knows, there may be other configurations possible, and it's easier to grok when there are no conditionals.

Maybe a richer comment would suffice, ie:

```
// Enable analog RGB LED control.  Must define 3 PWM pins separately,
// as KB_PIN_RGB_PWM_{A,B,C}.  Generally only larger boards like Arduino
// Mega have available pins for this.
#define KB_ENABLE_RGB_PWM 0
```