message: Adjust #if's for pybind_optional for pre-C++17 compilers
diff: * @brief Define a type_caster for akrzemi1::optional, which is used when the compiler doesn't have <optional> (e.g. on Apple).
  */
  -#ifdef __APPLE__
  +#if __cplusplus >= 201703L || (defined(_MSVC_LANG) && _MSVC_LANG >= 201703L)
  +
  +#include "pybind11/stl.h"
  +
  +#else
  #include "eos/cpp17/optional.hpp"
  #include "pybind11/stl.h"
  @@ -39,15 +43,13 @@ namespace detail {
  * @brief Type caster for akrzemi1::optional, which is used when the compiler doesn't have <optional> (e.g. on Apple).
  */
  template <typename T>
  -struct type_caster<akrzemi1::optional<T>> : optional_caster<akrzemi1::optional<T>> {};
  +class type_caster<akrzemi1::optional<T>> : optional_caster<akrzemi1::optional<T>>
  +{
  +};
  } /* namespace detail */
  } /* namespace pybind11 */
  -#else
  -
  -#include "pybind11/stl.h"
  -
  -#endif /* __APPLE__ */
  +#endif
  #endif /* EOS_PYBIND11_OPTIONAL_HPP_ */
  