Build a minimized Nix with MinGW

At this point many features are stripped out, but this works:

- Can run libnix{util,store,expr} unit tests
- Can run some Nix commands

Co-Authored-By volth <volth@volth.com>
Co-Authored-By Brian McKenna <brian@brianmckenna.org>

src/libutil/file-path.hh

@@ -0,0 +1,52 @@
+#pragma once
+///@file
+
+#include <optional>
+#include <filesystem>
+
+#include "types.hh"
+
+namespace nix {
+
+/**
+ * Paths are just `std::filesystem::path`s.
+ *
+ * @todo drop `NG` suffix and replace the ones in `types.hh`.
+ */
+typedef std::filesystem::path PathNG;
+typedef std::list<Path> PathsNG;
+typedef std::set<Path> PathSetNG;
+
+/**
+ * Stop gap until `std::filesystem::path_view` from P1030R6 exists in a
+ * future C++ standard.
+ *
+ * @todo drop `NG` suffix and replace the one in `types.hh`.
+ */
+struct PathViewNG : std::basic_string_view<PathNG::value_type>
+{
+    using string_view = std::basic_string_view<PathNG::value_type>;
+
+    using string_view::string_view;
+
+    PathViewNG(const PathNG & path)
+        : std::basic_string_view<PathNG::value_type>(path.native())
+    { }
+
+    PathViewNG(const PathNG::string_type & path)
+        : std::basic_string_view<PathNG::value_type>(path)
+    { }
+
+    const string_view & native() const { return *this; }
+    string_view & native() { return *this; }
+};
+
+std::string os_string_to_string(PathViewNG::string_view path);
+
+PathNG::string_type string_to_os_string(std::string_view s);
+
+std::optional<PathNG> maybePathNG(PathView path);
+
+PathNG pathNG(PathView path);
+
+}