gcc -lm

/usr/lib/gcc/x86_64-linux-gnu/7/../../../x86_64-linux-gnu/Scrt1.o: In function `_start':
(.text+0x20): undefined reference to `main'
collect2: error: ld returned 1 exit status

CC=gcc
CFLAGS=-I.
DEPS = main.h node.h print.h
OBJ = main.o node.o print.o

%.o: %.c $(DEPS)
    $(CC) -c -o $@ $< $(CFLAGS)
ukkonen: $(OBJ)
    $(CC) -o $@ $^ $(CFLAGS)

$ objdump -t main.o