clang: error: linker command failed with exit code 1 (use -v to see invocation) on OS X

Question

I am trying to compile the below C code:

    // (c) Partha Dasgupta 2009
    // permission to use and distribute granted.

    #define _GNU_SOURCE
    //#define _POSIX_C_SOURCE 200809L
    //#define _XOPEN_SOURCE 700
    #include <stdio.h>
    #include <stdlib.h>
    #include "threads.h"
    #include <unistd.h>

    typedef pthread_mutex_t lock_t;

    lock_t L1;

void init_lock(lock_t *l)
{
      pthread_mutex_init(l, NULL);
}

void lock(lock_t *l)
{
       pthread_mutex_lock (l); 
}

void unlock (lock_t *l)
{
       pthread_mutex_unlock (l); 
             pthread_yield();
}


void function_1(void)
{
    while (1){ 
        lock(&L1);
        printf("Beginning of CS: func 1\n");
        sleep(1);
        printf("End of CCS: func 1..\n");
        sleep(1);
    unlock(&L1);
    }
}    

void function_2(void)
{
    while (1){ 
        lock(&L1);
        printf("Beginning of CS: func 2\n");
        sleep(1);
        printf("End of CCS: func 2..\n");
        sleep(1);
    unlock(&L1);
    }
}    

void function_3(void)
{
    while (1){ 
       lock(&L1);
        printf("Beginning of CS: func 3\n");
        sleep(1);
        printf("End of CCS: func 3..\n");
        sleep(1);
    unlock(&L1);
    }
}    

int main()
{
    init_lock(&L1); 
    start_thread(function_1, NULL);
    start_thread(function_2, NULL);
    start_thread(function_3, NULL);

    while(1) sleep(1);

    return 0;
}

the threads.h file has the below code:

// (c) Partha Dasgupta 2009
// permission to use and distribute granted.

#include <pthread.h>

pthread_t start_thread(void *func, int *arg)
  {
   pthread_t thread_id;
   int rc;
      printf("In main: creating thread\n");
      rc = pthread_create(&thread_id, NULL, func, arg);
      if (rc){
         printf("ERROR; return code from pthread_create() is %d\n", rc);
         exit(-1);
        }
    return(thread_id);
   }

I am compiling it using the below in OS X terminal:

gcc -lpthread lock_test.c

the error message I am getting is:

lock_test.c:29:5: warning: implicit declaration of function 'pthread_yield' is invalid in C99 [-Wimplicit-function-declaration]
                         pthread_yield();
                         ^
1 warning generated.
Undefined symbols for architecture x86_64:
  "_pthread_yield", referenced from:
      _unlock in lock_test-ed5a79.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

I am unable to understand why I am not able to compile it. It works perfectly fine on a linux box, but not in OS X.

What should I do to get this compiled?