actually it is defined in the avr device header file, arduino is built right on top of avr gcc and you can get to gcc anytime (sometimes you have to when you want the most out of your hardware)
Code:
C:\arduino-0011\hardware\tools\avr\avr\include\avr>grep include iom168.h
#include <avr/iomx8.h>
C:\arduino-0011\hardware\tools\avr\avr\include\avr>grep PINC iomx8.h
#define PINC _SFR_IO8 (0x06)
/* PINC */
#define PINC6 6
#define PINC5 5
#define PINC4 4
#define PINC3 3
#define PINC2 2
#define PINC1 1
#define PINC0 0