[v3] c++0x std::ratio implementation

[Paolo Carlini]

> Updated patch. Formatting, use _GLIBCXX_USE_C99_STDINT_TR1,
> renamed template params.

Thanks. I'm finishing regtesting the patch + some minor local tweaks. While I do that, can you please double check that ratio_subtract is OK when _R2::num == INTMAX_MIN? I see an unconditional unary minus which in principle could overflow...

Thanks,
Paolo.