Use #define in pins_arduino.h to allow testing

[thelastoutpostworkshop]

Many pins definitions are static_const in pins_arduino.h which don't allow testing in code using #ifdef, just LED_BUILTIN seams to be that way, for example :
static const uint8_t SS = 34;
static const uint8_t MOSI = 35;
static const uint8_t MISO = 37;
static const uint8_t SDO = 35;
static const uint8_t SDI = 37;
static const uint8_t SCK = 36;

[thelastoutpostworkshop]

It could definitely help library makers!