makefile 变量赋值

immediate = deferred #延迟替换

immediate ?= deferred #条件变量赋值操作符，因为它仅仅在变量还没有定义的情况下有效

immediate := immediate #立刻替换变量

immediate += deferred or immediate

#

objects = main.o foo.o bar.o utils.o

# objects

+=

another.o

# 等同于

# objects = main.o foo.o bar.o utils.o

# objects := $(objects) another.o