tidbits: use C++ cmath functions

[tritao]

Summary

• Use std::isinf and std::isnan from <cmath> in tidbits.cpp.
• Preserve coin_isinf()'s documented -1 / +1 return contract with std::signbit.

Motivation

Some C++ environments do not expose the unqualified C isinf and isnan names when including <cmath>. Coin detects these functions through the C math.h header, but tidbits.cpp is C++ code, so using the std:: namespace is the portable form.