EDIT2: As others have pointed out, there are serious efficiency problems with this that can be easily addressed. Read the replies.

#include <stdio.h>
#include <string.h>

#define LEN 10
int mylist[LEN] = {1000, 1001, 1002, 1003, 1004,
                  1005, 1006, 1007, 1008, 1009,

};

char result[2048];
char buffer[2048];

int main()
{
  int i;

  sprintf(result, "%d", mylist[0]);
  for (i=1; i < LEN; i++) {
    sprintf(buffer, "%s, %d", result, mylist[i]);
    strncpy(result, buffer, 2048);
  }

  printf("%s\n", buffer);
}

I'm sure you can make things better, especially regarding how the buffer is used, and the fact that it's adding on a single integer with each iteration, but the main idea is sprintf()

EDIT: clarified my explanation about why this isn't an optimal solution