rework analogInputToDigitalPin definition

Marlin/Marlin.h

 # include "Arduino.h"
 #else
 # include "WProgram.h"
-  //Arduino < 1.0.0 does not define this, so we need to do it ourselves
+#endif
+
+// Arduino < 1.0.0 does not define this, so we need to do it ourselves
+#ifndef analogInputToDigitalPin
 # define analogInputToDigitalPin(p) ((p) + A0)
 #endif
 
 #endif
 
 #ifdef FILAMENT_SENSOR
-  extern float filament_width_nominal;  //holds the theoretical filament diameter ie., 3.00 or 1.75 
-  extern bool filament_sensor;  //indicates that filament sensor readings should control extrusion  
-  extern float filament_width_meas; //holds the filament diameter as accurately measured 
+  extern float filament_width_nominal;  //holds the theoretical filament diameter ie., 3.00 or 1.75
+  extern bool filament_sensor;  //indicates that filament sensor readings should control extrusion
+  extern float filament_width_meas; //holds the filament diameter as accurately measured
   extern signed char measurement_delay[];  //ring buffer to delay measurement
   extern int delay_index1, delay_index2;  //index into ring buffer
   extern float delay_dist; //delay distance counter
 
 #endif
 
-extern void calculate_volumetric_multipliers();
+extern void calculate_volumetric_multipliers();
+