CFLAGS = -Wall -std=gnu99 -O
CC = gcc
LDLIBS = -lm    # link with math library

default: out-hello.txt out-echo.txt out-math.txt

out-hello.txt: hello
	./hello > out-hello.txt

out-echo.txt: Makefile
	echo 'hello' > $@        # $@ here is actually the target: out-echo.txt

hello: hello.o
	$(CC) $(CFLAGS) hello.o -o hello

hello.o: hello.c
	$(CC) $(CFLAGS) -c $< -o $@    # here $< is the first ingredient: hello.c

.PHONEY:clean # clean is not a file but a phoney target
clean:
	rm -f out*.txt *.o
	find . -type f -executable -delete

out-math.txt: math    # math is built automaticall with built-in rules
	./math > $@ 

test:
	@echo '$$(CC)=' $(CC)
	@echo '$$(CFLAGS)=' $(CFLAGS)
	@echo 'CFLAGS=' CFLAGS