[Patch] Use variadic templates for tr1::is_function

[Paolo Carlini]

Doug Gregor wrote:

> That doesn't work because one can't return a function from a  
> function. Of course, one can't have an object with function type (an  
> object can have reference-to-function or pointer-to-function type), ...

Ok, now I see. In short, poor me, in my stupid mumblings of this after 
noon I was not using the concept of callable type and all that...

Thanks again,
Paolo.