Bug in C++ interface (comparison between rational and double)

[Abramo Bagnara]

The comparison between mpz_class and double works fine with +inf and
-inf, while the comparison between mpq_class and double gives a floating
point exception, due to inappropriate mpq_set_d call in eval specialization.

I see two way to fix this:

1) add support for comparison between mpq and double in C interface and
use that in gmpxx.h

2) add infinity (and nan) check in comparison eval specializations in
gmpxx.h

IMHO 1) gives a more complete solution. If you want we're willing to
write the (trivial) patch.

-- 
Abramo Bagnara

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