问如何使Makefile编译一个目录中的多个c文件和h文件

EN

all: 5-1 5-2 5-3

5-1: 5-1.c getch.c
    gcc -Wall -v -g -o 5-1 5-1.c getch.c

5-2: 5-2.c getch.c
    gcc -Wall -g -o 5-2 5-2.c getch.c

// The issues happens here. I does not generate string.o
// Also, I feel the format of 5-3 does not looks correct.
// I do not know how to write it here.
5-3: 5-3.o string.o
    gcc -Wall -g -o 5-3 5-3.o string.o

string.o: string.c string.h
    gcc -Wall -g -o string.c string.h types.h

clean:
    rm -f 5-1 5-2 5-3
    rm -rf *.dSYM

#include "string.h"

char *strcat(char *dst, const char *src)
{
    char* cp = dst;

    while(*cp) {
        cp++;
    }

    while(*src)
    {
        *cp++ = *src++;
    } 

    return dst;
}

#ifndef _STRING_H
#define _STRING_H

#include <stdio.h>
#include "types.h"

#ifndef NULL
#define NULL 0
#endif

char   *strcat(char *, const char *);

#endif /* _STRING_H */

#ifndef _SIZE_T_DEFINED
#define _SIZE_T_DEFINED
    typedef unsigned int uint32_t;
#endif