override var += -g #通过命令行传递的参数，不能覆盖var的值
sources = f1.c f2.c
define hello
@echo "test define"
@echo "test define hello"
endef
test:foo
$(call hello) #调用define定义的hello语句块。
@echo "test"
foo:
@echo "foo"
@echo $(var) #make var=-v ==> -g -v
#函数使用
@echo $(findstring a,a b c) # a $(findstring <find>,<in>)
@echo $(patsubst %/,%,test/foo/) #test/foo
@echo $(subst lh,LH, lhxlhx) #LHxLHx
@echo $(strip a b c ) #a b c 返回被去掉空格的字符串值
@echo $(filter %.c %.s,f1.c f2.o f3.s) #f1.c f3.s
@echo $(filter-out %.c %.s,f1.c f2.o f3.s) #f2.o
@echo $(sort c1 a2 b3) #a2 b3 c1
@echo $(word 2, lhx ai yll) #ai
@echo $(wordlist 2, 3, lhx ai yll) #ai yll
@echo $(words lhx ai yll) #3
@echo $(firstword lhx ai yll) #lhx
@echo $(dir src/foo.c hacks) #src/ ./
@echo $(notdir src/foo.c hacks) #foo.c hacks
@echo $(suffix src/foo.c src-1.0/bar.c hacks) #.c .c
@echo $(basename src/foo.c src-1.0/bar.c hacks) #src/foo src-1.0/bar hacks
@echo $(addsuffix .c,foo bar) #foo.c bar.c
@echo $(addprefix src/,foo bar) #src/foo src/bar
@echo $(join aaa bbb , 111 222 333) #aaa111 bbb222 333
@echo $(foreach n,a b c,$(n).o) #a.o b.o c.o
@echo $(shell echo "hello") #hello
###@echo $(error error is test) #Makefile:34: *** error is test. Stop.
@echo $(sources:.c=.d) #f1.d f2.d
