Allow wchar_t constructors of fstreams on Windows when using libstdc++

[rikyoz]

Hi!
Currently, when using libstdc++ on Windows (e.g., MinGW), ghc::filesystem's streams open the file using the narrow string representation of the path.

However, recent versions of libstdc++ have added support for opening file streams directly using wchar_t paths (gcc-mirror/gcc@b029235).

Using the wide/native string type for opening the file streams is preferred on Windows, especially for supporting paths with Unicode characters.

This PR introduces a new #define called GHC_HAS_FSTREAM_OPEN_WITH_WCHAR (I could not find any better name 😅).
It is defined when the OS is Windows and either we are not using libstdc++ (e.g., libc++ or Microsoft's STL) or the libstdc++ supports opening files using wchar_t paths.
For checking this latter case, I used the same preprocessor check done in libstdc++ (e.g., https://github.com/gcc-mirror/gcc/blob/b02923592a55b52d796656c5b4f34a964a2604a9/libstdc%2B%2B-v3/include/std/fstream#L536)

[gulrak]

Nice, I like it and the name is good, at least one knows what it does. 😄

[rikyoz]

Great, thank you!

By the way, I've also found and implemented a workaround for opening file streams using wide string paths when using old versions of MinGW that do not support GHC_HAS_FSTREAM_OPEN_WITH_WCHAR.

This should fix the support for even more versions of MinGW, which currently can't correctly open files with Unicode characters in the path.

If it's not a problem, I might open another pull request after this is closed!

[gulrak]

Thanks again for the PR!