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/windows/file-path.cc

@@ -0,0 +1,52 @@
+#include <algorithm>
+#include <codecvt>
+#include <iostream>
+#include <locale>
+
+#include "file-path.hh"
+#include "file-path-impl.hh"
+#include "util.hh"
+
+namespace nix {
+
+std::string os_string_to_string(PathViewNG::string_view path)
+{
+    std::wstring_convert<std::codecvt_utf8_utf16<wchar_t>> converter;
+    return converter.to_bytes(PathNG::string_type { path });
+}
+
+PathNG::string_type string_to_os_string(std::string_view s)
+{
+    std::wstring_convert<std::codecvt_utf8_utf16<wchar_t>> converter;
+    return converter.from_bytes(std::string { s });
+}
+
+std::optional<PathNG> maybePathNG(PathView path)
+{
+    if (path.length() >= 3 && (('A' <= path[0] && path[0] <= 'Z') || ('a' <= path[0] && path[0] <= 'z')) && path[1] == ':' && WindowsPathTrait<char>::isPathSep(path[2])) {
+        PathNG::string_type sw = string_to_os_string(
+            std::string { "\\\\?\\" } + path);
+        std::replace(sw.begin(), sw.end(), '/', '\\');
+        return sw;
+    }
+    if (path.length() >= 7 && path[0] == '\\' && path[1] == '\\' && (path[2] == '.' || path[2] == '?') && path[3] == '\\' &&
+               ('A' <= path[4] && path[4] <= 'Z') && path[5] == ':' && WindowsPathTrait<char>::isPathSep(path[6])) {
+        PathNG::string_type sw = string_to_os_string(path);
+        std::replace(sw.begin(), sw.end(), '/', '\\');
+        return sw;
+    }
+    return std::optional<PathNG::string_type>();
+}
+
+PathNG pathNG(PathView path)
+{
+    std::optional<PathNG::string_type> sw = maybePathNG(path);
+    if (!sw) {
+        // FIXME why are we not using the regular error handling?
+        std::cerr << "invalid path for WinAPI call ["<<path<<"]"<<std::endl;
+        _exit(111);
+    }
+    return *sw;
+}
+
+}