关于makefile的dependency的问题，请教一个makefile的用法

[max_xy]

all :test
test:hello.o
gcc -o test hello.o
hello.o : hello.c
gcc -c hello.c
clean :
rm *.o
TAGS:
@rm -f $@
find . -name '*.[ch]' | xargs etags -a
.makefirst:
@echo Making dependencies: please wait...
@touch .makefirst
%.h::
@echo Something wrong... deleting dependencies.
@-rm -f .makefirst

-include .makefirst

请问
这里TAGS和.makefirst，%.h::都在什么情况下执行？