Don't allow __VA_OPT__ to be detected by #ifdef.

More study has discovered this to not actually be useful: because
current C++20 implementations reject `#ifdef __VA_OPT__`, this can't
really be used as a feature-test mechanism. And it's not too hard to
detect __VA_OPT__ without this, for example:

  #define THIRD_ARG(a, b, c, ...) c
  #define HAS_VA_OPT(...) THIRD_ARG(__VA_OPT__(,), 1, 0, )
  #if HAS_VA_OPT(?)

Partially reverts 0436ec21.