varför får jag fel? (Visual studio) "Error LNK 2019"

1>------ Build started: Project: afdsfs, Configuration: Debug Win32 ------
1>Build started 2012-11-19 18:57:11.
1>PrepareForBuild:
1>  Creating directory "C:\afdsfs\Debug\".
1>InitializeBuildStatus:
1>  Creating "Debug\afdsfs.unsuccessfulbuild" because "AlwaysCreate" was specified.
1>ClCompile:
1>  kiyfdykyd.cpp
1>c:\afdsfs\afdsfs\kiyfdykyd.cpp(84): warning C4244: 'argument' : conversion from 'time_t' to 'unsigned int', possible loss of data
1>c:\afdsfs\afdsfs\kiyfdykyd.cpp(155): warning C4996: 'scanf': This function or variable may be unsafe. Consider using scanf_s instead. To disable deprecation, use _CRT_SECURE_NO_WARNINGS. See online help for details.
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\stdio.h(304) : see declaration of 'scanf'
1>c:\afdsfs\afdsfs\kiyfdykyd.cpp(203): warning C4996: 'scanf': This function or variable may be unsafe. Consider using scanf_s instead. To disable deprecation, use _CRT_SECURE_NO_WARNINGS. See online help for details.
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\stdio.h(304) : see declaration of 'scanf'
1>kiyfdykyd.obj : error LNK2019: unresolved external symbol "int __cdecl play_again(void)" (?play_again@@YAHXZ) referenced in function _main
1>C:\afdsfs\Debug\afdsfs.exe : fatal error LNK1120: 1 unresolved externals
1>
1>Build FAILED.
1>
1>Time Elapsed 00:00:00.27
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

/*
 *  File    : nim.c
 *  Program : Nim game
 *  Author  : ...
 */

#include<stdio.h>
#include<stdlib.h>
#include<time.h>
#include<math.h>


#define MAX_COINS 13

const int HUMAN = 0;
const int COMPUTER = 1;

/* ------------- IO --------------- */

/*
 * human_choice
 * Get human choce as an int from stdin.
 * Clears stdin.
 * in: pile
 * out: int-value in range 1-3 (and also less than pile)
 */
int human_choice(int pile);

/*
 * write_winner
 * Write winner as a string on stdout.
 * in: Values HUMAN or COMPUTER.
 * out:
 */
void write_winner( int player );

/*
 * play_again
 * Ask human if he/she wants to play
 * another round. If 'n' or 'N' selected
 * (single char) return false else true.
 * Clears stdin.
 * in:
 * out: true or false
 */
int play_again();

/* ---------------- Logic ----------------*/

/*
 * computer_choice
 * Get computers choice (including some AI,
 * if 4 coins or less left, function makes a
 * smart choice else random).
 * in: pile
 * out: int-value in range 1-3 (and also less than pile)
 */
int computer_choice(int pile);

/*
 * toggle
 * Switches player, if HUMAN in COMPUTER out
 * etc.
 * in:  actual player
 * out: next player
 */
int toggle( int player );

/* --------------------- Utilities -------------*/
void clear_stdin();

/***************************************************
 *
 *    MAIN
 *
 ***************************************************/
int main()
{
    int pile,			/* This is how many coins we have */
    player,			/* Who is playing? */
    n_coins,            /* Number of coins taken */
    forts = 0;      /* Do the player want to continue?*/
    
    srand (time(0) );		/* Setup random */
    
    printf("Välkommen till NIM by Oskar Selberg & Johan Strand\n");
    
    pile = MAX_COINS;		/* Set start values (= init) */
    player = HUMAN;
    
    /*
     *  Program main loop
     */
        
    while( forts == 0 ) {
        int truth = 0;/*instead of _Bool*/
        
        printf("Det ligger %d  mynt i högen\n", pile );
        
        if( player == HUMAN ){
            n_coins = human_choice(pile);
        }
        else {
            n_coins = computer_choice(pile);
            printf("- Datorn tog %d\n", n_coins);
        }
        pile -= n_coins;
        player = toggle( player );
        
        if( pile <= 1 ){
            write_winner( player );
            printf("\navslutat spel");
            truth = play_again();
            if (truth == 1) {
                pile = MAX_COINS;
                printf("\n");
                player = HUMAN;
            }
            else {
                forts = 1;
                break;
            }
        }
    }

    /*
     * end main loop
     */
    while ( forts == 1 ) {
        printf("\n\nAvslutar spel\n");
        return 0;
    }
}

/******************************************************
 *
 *  DEFINITIONS
 *
 ******************************************************/


void clear_stdin()
{
    while( getchar() != '\n' ){
        ;
    }
}

/*hur många mynt tar spelaren*/
int human_choice(int pile) {
    int n_coins = 0;
    int x = 0;
    printf("\nHur många mynt vill du ta?(mellan 1 och 3):");
    while (x == 0) {
        scanf("%i", &n_coins);
        if (n_coins>3 || n_coins<0 || n_coins>=pile ) {
            printf("du valde ett ogiltligt antal mynt!\nHur många mynt vill du ta?");
        }
        else {
            x=1;
        }
    }
    clear_stdin();
    return n_coins;
}

/*hur många mynt datorn tar*/
int computer_choice(int pile) {
    int n_coins = 0;
    
    if (pile == 4) {
        n_coins = 3;
    }
    else if (pile == 3) {
        n_coins = 2;
    }
    else if (pile == 2) {
        n_coins = 1;
    }
    else {
        n_coins = (rand() % 3) + 1;
    }
    return n_coins;
}

/*skriv ut vinnaren*/
void write_winner(int player ) {
    if (player == 0) {
        printf("Datorn vann över dig!\n");
    }
    else if (player == 1) {
        printf("Du vann över Datorn!\n");
    }
}

/*vill spelaren spela igen*/
int play_again( int truth) {
    char fort;
    int x = 0;
    
    while (x == 0) {
        printf("\nvill du spela igen? Y/N: ");
        scanf("%c", &fort);
    
        if ((fort == 'Y' )||( fort == 'y' )) {     /*han vill fortsätta*/
            x = 1;
            break;
            
        }
        else if ((fort == 'N' )|| (fort == 'n' )) {     /*han vill inte fortsäta*/
            x = 0;
            break;
        }
        else {
            printf("du har matat en ett okänt tecken, försök igen\n");      /*inte giltligt värde på inmatning*/
            clear_stdin();
        }
    }
    return x;
}

/*vems tur är det?*/
int toggle( int player ) {
    if (player == 1) {
        player = 0;
    }
    else {
        player = 1;
    }
    return player;
}