Hi,

Marc kindly contributed the below rewrite of std::ratio_add (and 
ratio_less), algorithmically derived from the macro __udiv_qrnnd_c in 
gcc's longlong.h, which amounts to the best behavior in terms of 
accepted inputs: a pair of std::ratio is *never* rejected unless the 
resulting std::ratio would not be legal.

Tested x86_64-linux multilib, committed to mainline.

Paolo.

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