bct_p *is* used in loop.c:strength_reduction

[Joe Buck]

> `unused'
>      This attribute, attached to a variable, means that the variable is
>      meant to be possibly unused.  GNU CC will not produce a warning
>      for this variable.
> 
> I do not know what "meant to be possibly" is intended to convey.  This is
> intended for cases where some compilations will not use the variable?

In practice, it just says "don't warn if this variable is unused" and
the description should say so.