Trying to make sense of the coding style in Arduino libraries. (Particularly the Radiohead library)
They're not defined. If you notice that whole block is wrapped in a #ifdef which will be false unless you're on some specific platform, where the platform defines those macros.

Everything else uses the last little bit:

pinMode(_txPin, OUTPUT);
pinMode(_rxPin, INPUT);
pinMode(_pttPin, OUTPUT);


You only need to care about that bit - nothing else is of any concern unless you're using a "RH_PLATFORM_GENERIC_AVR8" (whatever that is).