Allow attachInterrupt without pinMode (for AVR compatibility)

teensy3/pins_teensy.c

 	}
 	mask = (mask << 16) | 0x01000000;
 	config = portConfigRegister(pin);
-
+	if ((*config & 0x00000700) == 0) {
+		// for compatibility with programs which depend
+		// on AVR hardware default to input mode.
+		pinMode(pin, INPUT);
+	}
 #if defined(KINETISK)
 	attachInterruptVector(IRQ_PORTA, port_A_isr);
 	attachInterruptVector(IRQ_PORTB, port_B_isr);