Why is the ternary operator used to define 1 and 0 in a macro?

You are correct, in C it is tautologous. Both your particular ternary conditional and (1 > 0) are of type int.

But it would matter in C++ though, in some curious corner cases (e.g. as parameters to overloaded functions), since your ternary conditional expression is of type int, whereas (1 > 0) is of type bool.

My guess is that the author has put some thought into this, with an eye to preserving C++ compatibility.