Add SPI_HAS_TRANSACTION define

Modern Arduino cores require to implement beginTransaction, so we can always define SPI_HAS_TRANSACTION.

Fixes adafruit/Adafruit-GFX-Library#242

Author: facchinm

Diff for: api/HardwareSPI.h

@@ -22,6 +22,8 @@
 #include <inttypes.h>
 #include "Stream.h"
 
+#define SPI_HAS_TRANSACTION
+
 namespace arduino {
 
 typedef enum {