change analog reference options from enum to #define constants

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

Locate the platform-specific analog reference declarations and usages, starting with the Zero example shown in the issue. Compare all supported reference choices and verify that the declarations use compile-time-testable constants and that affected platform code still compiles.

Written by the indexing model from the issue text.