box: Use isinf() and signbit() when isinff() is not available

[ebassi]

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Instead of using bytewise comparison, use isinf() and hope it works correctly with single precision floats on the target platform; additionally, since the negative return value for negative infinities is a GNU extension, use signbit() to check if the number is a positive or negative infinity.

Proposed changes:

• change the fallback path for the is_empty() and is_infinity() methods of graphene_box_t