Use GHC::filesystem for std::filesystem compatibility

[dpogue]

This brings in https://github.com/gulrak/filesystem as a "polyfill" for the C++17 filesystem API, particularly on older macOS versions where it's not part of the system C++ library. This should resolve #1065.

If possible, it will use the std::filesystem API, but when targeting macOS versions without support it will use the GHC implementation instead.

This uses the latest master branch of gulrak/filesystem because the current tagged release doesn't work properly on macOS.

I opted to just drop this in the common folder for ease of use, since it's a single file and that's what was done with pffft, but I could try to put it in a top-level vendor folder similar to fmt if that's the preferred approach.

[kcat]

I'm not very keen on adding a custom implementation of an API that's part of the required standard, for an OS that's over 7 years old and didn't update to improve support. Simple workarounds would be fine, but a full implementation seems much. Especially since the next (non-point) release will be increasing the standard requirement to C++20, so won't be useful just after being added.

Is this really all that's needed to make it work for the system?

[dpogue]

Agree that a full filesystem implementation is probably overkill, particularly given how minimal the use of it is in openal-soft, but this seemed like the option that involved the least code changes by swapping in something with the same API.

The issue on macOS is that the C++ runtime is provided by the system and doesn't get updated. So even if you build with a newer compiler version, you can't use APIs that weren't part of the system C++ runtime of the target system. It's theoretically possible to do static linking, but that's very uncommon and potentially introduces incompatibilities with system frameworks like CoreAudio. Previous attempts at mixing C++ runtimes have resulted in issues like pointers being malloc'ed with one runtime and free'd in another and throwing errors.

Unfortunately it comes down to implementation details which parts of the C++ standard are compiled away and which parts involve additions to the runtime, but filesystem is definitely one that isn't available on older macOS versions.