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/users.cc

@@ -0,0 +1,50 @@
+#include "util.hh"
+#include "users.hh"
+#include "environment-variables.hh"
+#include "file-system.hh"
+#include "windows-error.hh"
+
+#define WIN32_LEAN_AND_MEAN
+#include <windows.h>
+
+namespace nix {
+
+std::string getUserName()
+{
+    // Get the required buffer size
+    DWORD size = 0;
+    if (!GetUserNameA(nullptr, &size)) {
+        auto lastError = GetLastError();
+        if (lastError != ERROR_INSUFFICIENT_BUFFER)
+            throw WinError(lastError, "cannot figure out size of user name");
+    }
+
+    std::string name;
+    // Allocate a buffer of sufficient size
+    //
+    // - 1 because no need for null byte
+    name.resize(size - 1);
+
+    // Retrieve the username
+    if (!GetUserNameA(&name[0], &size))
+        throw WinError("cannot figure out user name");
+
+    return name;
+}
+
+Path getHome()
+{
+    static Path homeDir = []()
+    {
+        Path homeDir = getEnv("USERPROFILE").value_or("C:\\Users\\Default");
+        assert(!homeDir.empty());
+        return canonPath(homeDir);
+    }();
+    return homeDir;
+}
+
+bool isRootUser() {
+    return false;
+}
+
+}