Makefile of calculator

override CFLAGS += -g -Wall -Wextra
override LDFLAGS +=
override LDLIBS += -lm
all: z
z: y.tab.c lex.yy.c
    gcc $(CFLAGS) $(LDFLAGS) $(LDLIBS) -o $@ $<
y.tab.c: z.y
    bison -dy $<
y.tab.h: y.tab.c
lex.yy.c: z.lex y.tab.h
    flex $<
clean:
    rm -f z y.tab.c y.tab.h lex.yy.c