support copysign jet

libc++ (default on macOS) uses the copysign function for std::complex
multiplication which causes compilation errors such as these if
std::complex is combined with jets:

/Applications/Xcode_12.4.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/complex:607:19: error: no matching function for call to 'copysign'
            __a = copysign(__libcpp_isinf_or_builtin(__a) ? _Tp(1) : _Tp(0), __a);
                  ^~~~~~~~
foo.hpp:218:39: note: in instantiation of function template specialization 'std::__1::operator*<ceres::Jet<double, 3> >' requested here
    std::complex<Scalar> result = lhs * rhs;

copysign is also useful in other situations where the sign is required
allowing it to compute without branching.

Change-Id: I84b2d23374f1bf3bae32833016bb686d97d74054