rework analogInputToDigitalPin definition

Marlin/Marlin.h

@@ -30,7 +30,10 @@
 # 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
 
@@ -270,3 +273,4 @@ extern void digipot_i2c_init();
 #endif
 
 extern void calculate_volumetric_multipliers();
+