Add unlikely() and likely() wrappers in gnu C++

gistfile1.cpp

/* from the cython compiler */

#ifdef __GNUC__
  /* Test for GCC > 2.95 */
  #if __GNUC__ > 2 || (__GNUC__ == 2 && (__GNUC_MINOR__ > 95))
    #define likely(x)   __builtin_expect(!!(x), 1)
    #define unlikely(x) __builtin_expect(!!(x), 0)
  #else /* __GNUC__ > 2 ... */
    #define likely(x)   (x)
    #define unlikely(x) (x)
  #endif /* __GNUC__ > 2 ... */
#else /* __GNUC__ */
  #define likely(x)   (x)
  #define unlikely(x) (x)
#endif /* __GNUC__ */