ARDUINO_MAIN defined on wrong place

[martinstingl]

I tried to compile the Arduino Core to a static library for an ATmega328p.
In order to do that I excluded the main.cpp from the build.
Compiling gives an error because DDRA etc. is not defined (there is no Port A on an ATmega328p).

The problem is that ARDUINO_MAIN is always defined in wiring_digital.c which leads to errors with pins_arduino.h because &DDRA etc. cannot be found.

Wouldn't it be better to move the line #define ARDUINO_MAIN to main.cpp?