C :: Program That Converts Decimal To Any Base (mostly)

I had the idea to write a program that would convert a decimal integer into any base up to 36 (there are no letters after that) so I decided to give it a try.

Code:
#include <stdio.h>
#include <stdlib.h>
char letters(int r); // prototypes function for converting 10's to A's and so on, base 11+ will work
int main() {

int base;
int d; //declares decimal integer
int d_clone; // clones d for a loop

[Code] ......