[PATCH v2] Fix Wundef warning for MEMCPY_OK_FOR_FWD_MEMMOVE

[Roland McGrath]

> --- a/sysdeps/generic/memcopy.h
> +++ b/sysdeps/generic/memcopy.h
> @@ -148,4 +148,6 @@ extern void _wordcopy_bwd_dest_aligned (long int, long int, size_t) __THROW;
>  /* Threshold value for when to enter the unrolled loops.  */
>  #define	OP_T_THRES	16
>  
> +#define MEMCPY_OK_FOR_FWD_MEMMOVE 0

This name is pretty self-explanatory, but as a general rule a
sysdeps/generic file like should have copious comments that make clear what
the protocol for other sysdeps variants is and what each symbol means.