change analog reference options from enum to #define constants

[WestfW]

Currently, on platforms that have multiple choices for source of the analog reference voltage, the possible choices are defined in an enum (this example from Zero):

typedef enum _eAnalogReference
{
  AR_DEFAULT,
  AR_INTERNAL,
  AR_EXTERNAL,
  AR_INTERNAL1V0,
  AR_INTERNAL1V65,
  AR_INTERNAL2V23
} eAnalogReference ;

It has been suggested that this is less useful than #define values, because it can not checked at compile time (in attempts at platform-independent code) with #ifdef

http://forum.arduino.cc/index.php?topic=387409.msg2671892#msg2671892