make gcc error reporting better readable

Originally, gcc would report that error is present on line 5432
in allsrc.cpp. This would not help much. We can use gcc preprocessor
marks to make it report line number of original file together with
its file name.

src/Makefile

@@ -742,7 +742,8 @@ extcoff: $(TARGET).elf
 
 # Concatenate all sources files in one big file to optimize size
 allsrc: $(CPPSRC)
-	cat $(CPPSRC) > allsrc.cpp
+	@echo -n > allsrc.cpp
+	for f in $(CPPSRC); do echo "# 1 \"$$f\"" >> allsrc.cpp; cat "$$f" >> allsrc.cpp; done
 	
 remallsrc:
 	$(REMOVE) allsrc.cpp