Add core/FileSystem.hpp for accessing std::filesystem

This file enables access to std::filesystem as `fs` namespace
or injects third-party drop-in replacement on build configs
where std::filesystem is not available.

src/openrct2/core/FileSystem.hpp

@@ -0,0 +1,40 @@
+/*****************************************************************************
+ * Copyright (c) 2014-2019 OpenRCT2 developers
+ *
+ * For a complete list of all authors, please refer to contributors.md
+ * Interested in contributing? Visit https://github.com/OpenRCT2/OpenRCT2
+ *
+ * OpenRCT2 is licensed under the GNU General Public License version 3.
+ *****************************************************************************/
+
+/// @file
+/// This file enables access to std::filesystem as `fs` namespace
+/// or injects third-party drop-in replacement on build configs
+/// where std::filesystem is not available.
+
+#pragma once
+
+// Find out if std::filesystem is available:
+#if defined(_MSC_VER) // Visual Studio supports <filesystem>
+#    define HAVE_STD_FILESYSTEM 1
+#elif defined(__APPLE__) // XCode has the header, but reports error when included.
+#    define HAVE_STD_FILESYSTEM 0
+#elif defined(__has_include) // For GCC/Clang check if the header exists.
+#    if __has_include(<filesystem>)
+#        define HAVE_STD_FILESYSTEM 1
+#    else
+#        define HAVE_STD_FILESYSTEM 0
+#    endif
+#else // By default assume not supported.
+#    define HAVE_STD_FILESYSTEM 0
+#endif
+
+#if HAVE_STD_FILESYSTEM
+#    include <filesystem>
+namespace fs = std::filesystem;
+#else
+#    include "../thirdparty/filesystem.hpp"
+namespace fs = ghc::filesystem;
+#endif
+
+#undef HAVE_STD_FILESYSTEM // Not needed any more, don't make it public.