Makefile 變數指定

// 若指定值使用其他變數，此變數會連帶更動
VARIABLE = value

// 若指定值使用其他變數，此變數不會連帶更動

VARIABLE := value  

// 若變數未設定值時才給值

VARIABLE ?= value 

 

// 增加字串到原有變數

VARIABLE += value

References :

gnu make - Makefile variable assignment - Stack Overflow

make: `test' is up to date.

To force rebuild on these kind of not-file-related targets, you should make them phony as follows:

$ vi Makefile
.PHONY: all test clean

References :
makefile - Why does make think the target is up to date? - Stack Overflow