is print_ptree a dependency?
FLAGS = -Wall -Werror -std=c99
DEPENDENCIES = ptree.h

all: print_ptree

print_ptree: print_ptree.o ptree.o
      gcc ${FLAGS} -o $@ $^

%.o: %.c ${DEPENDENCIES}
      gcc ${FLAGS} -c $<

clean: 
      rm -f *.o print_ptree