Code:
#include<stdio.h>

//Enable first program
#define run1
//Enable second program
//#define run2

//First program
#ifdef run1
int main()
{
  printf("I am from main in run1 section");
  return 0;
}
#endif

//second Program
#ifdef run2
int main()
{
  printf("I am main in run2 section");
  return;
}
#endif