Xcode problem: ld: symbol(s) not found for architecture x86_64

[ar2007]

Hello!
I tried to create a header, where I want to insert the functions that usually use, but when I try to compile with Xcode 5.1, I get this error:

#ifndef test_header_prova_degli_HEADER_h
#define test_header_prova_degli_HEADER_h

extern int array[ 3 ];

int funzione_1( int a, int b );

void funzione_2( int c );

#endif

################################################

#include "prova_degli_HEADER.h"

int array[ 3 ] = { 0 , 1 , 2 };

int funzione_1( int a , int b )
{
   return ( a + b );
}

void funzione_2( int c )
{
   std::cout << " result is: " << c << std::endl;
}


#################################################

#include <iostream>
#include "prova_degli_HEADER.h"

int main()
{
	for (int i = 0; i < 3; ++i )
	{
		funzione_2( funzione_1( i , 2*i ) );		
	}
	return 0;
}

#################################################

  ld: symbol(s) not found for architecture x86_64
  clang: error: linker command failed with exit code 1 (use -v to see invocation)

I searched on google but there seems to be a similar problem, or better the reasons that can cause this error are different and can not seem to solve the problem.

any suggestions?
Thanks in advance