throw specs on Standard destructors

[Benjamin Kosnik]

>>> Third, any code that depends on tight throw specs in libstdc++
>>> would not be portable to other implementations. 

>Any code that depends on tight throw specs of Standard-conformant
>implementations would not be portable to GCC.

... which is far more of an issue, I think. 

17.4.4.8 -3- No destructor operation defined in the C++ Standard Library
will throw an exception. [...] An implementation may strengthen this
implicit exception-specification by adding an explicit one.

I still think the example code, as posted, is conformant. If it can't
compile, there's a problem. 

-benjamin