How to debug makefile ?

1. info, error

SOME_VARIABLE := $(SOME_OTHER_VARIABLE)/$(ANOTHER_VARIABLE)
$(info SOME_VARIABLE = $(SOME_VARIABLE)) -> 相當於 echo $(error SOME_VARIABLE = $(SOME_VARIABLE)) -> echo + exit

2. make -n -p

可以把所有的 target 展開，方便追找不到的 rule