Fix for Windows, both oF v0.12.1 / oF v0.12.0

[ffunatsu]

Confirmed working both macOS / Windows oF v0.12.1

( I also confirmed that without .string() not works on Windows. )

[ffunatsu]

Now it's working also on oF v0.12.0 ( the same one to #2. )

[NickHardeman]

Thank you!

[ffunatsu]

Thank you for merging.

As an aside, I myself was troubled by the mixing of std::optional and boost::optional in other projects, so creating a custom class might be a good idea.

[ffunatsu]

I recently confirmed #include <optional> is available on oF 0.12.0 default settings. So in this case, true problem was just not included <optional>.

Now it's OK that now using another optional class. FIY, I'm using this in my another project to make std::optional compatible as possible.

#pragma once

#if __cplusplus >= 201703L
#if __has_include(<optional>)

#define USE_STD_OPTIONAL

#endif // __has_include(<optional>)
#endif // __cplusplus >= 201703L

#ifdef USE_STD_OPTIONAL
#include <optional>
#else
#include "nonstd/optional.hpp"
#endif // USE_STD_OPTIONAL

namespace opt { // as you like

#ifdef USE_STD_OPTIONAL

    using std::optional;
    using std::nullopt;

    static std::nullopt_t none = std::nullopt;

#else

    using nonstd::optional;
    using nonstd::nullopt;

    static nonstd::nullopt_t none = nonstd::nullopt;

#endif // USE_STD_OPTIONAL

}

// now you can use optional as opt::optional