Visual Studio: Require new preprocessor

[martinfinke]

The conversion between enum and string doesn't seem to work using Visual Studio. The reason is that the preprocessor behaves incorrectly, so the string for an enum value becomes "WISE_ENUM_IMPL_IIF_1 (WISE_ENUM_IMPL_FIRST_ARG (SomeEnumValue, 0), (SomeEnumValue, 0))" instead of "SomeEnumValue".

This can be solved by enabling the new conforming preprocessor. wise_enum could check this at compile-time:

#ifdef _MSC_VER
#ifdef _MSVC_TRADITIONAL
#if _MSVC_TRADITIONAL
#error "The traditional, non-standard preprocessor is not supported. From Visual Studio 15.8 through 16.4, add the '/experimental:preprocessor' compiler option. From Visual Studio 16.5 onwards, add the '/Zc:preprocessor' compiler option."
#endif
#else
// _MSVC_TRADITIONAL is not defined, check from_string/to_string using a test enum:
namespace detail {
    enum class TestEnum { SomeTestValue };
}
static_assert(detail::strcmp(to_string(detail::TestEnum::SomeTestValue), "SomeTestValue") == 0
                  && from_string<detail::TestEnum>("SomeTestValue") == detail::TestEnum::SomeTestValue,
              "The traditional, non-standard preprocessor is not supported. From Visual Studio 15.8 through 16.4, add the '/experimental:preprocessor' compiler option. From Visual Studio 16.5 onwards, add the '/Zc:preprocessor' compiler option.");
#endif // _MSVC_TRADITIONAL
#endif // _MSC_VER

The static_assert is a fallback for Visual Studio < 15.8, where _MSVC_TRADITIONAL isn't defined at all.

I thought I'd share this, in case someone else runs into it. This info could also be added to the README.