#include <stdio.h>

void main(int argc, char *argv[]){
int inx;

for(inx=1;inx<=100;inx++) {
  printf("Inx equals %d ",inx);
  if ( inx % 3 == 0 ) {
    printf("fizz");
  }
  if ( inx % 5 == 0 ) {
    printf("buzz");
  }
  printf("\n");
}
}

Version 2 with function
File fizzzbuzz.c

#include <stdio.h>

char *fizz(int);
void main(int argc, char *argv[]){
int inx;

for(inx=1;inx<=100;inx++) {
  printf("Inx equals %d ",inx);
  printf(fizz(inx));
  if ( inx % 5 == 0 ) {
    printf("buzz");
  }
  printf("\n");
}
}

File fizz.c

char *fizz(int Val) {
if ( Val % 3 == 0 ) {
   return ("fizz");
}
else {
   return ("");
}
}

File fizzbuzz.mak (From an AIX xlc 6 machine)

clean : 
	rm -f  *z.o
	rm -f  fizzbuzz
	rm -f  fizzbuzz.err
fizz.o : fizz.c
	  cc_r -c -O -o fizz.o fizz.c >> fizzbuzz.err 2>&1

fizzbuzz: fizz.o fizzbuzz.o
	 xlc_r fizz.o fizzbuzz.o -o fizzbuzz >> fizzbuzz.err 2>&1