added informative error message for people building 0.6 for atmega168 (impossible)

main.c

@@ -31,6 +31,10 @@
 #include "settings.h"
 #include "wiring_serial.h"
 
+#ifndef __AVR_ATmega328P__
+#  error "As of version 0.6 Grbl only supports atmega328p. If you want to run Grbl on an 168 check out 0.51 ('git co v0_51')"
+#endif
+
 int main(void)
 {
   sp_init();