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/users.hh

@@ -3,16 +3,20 @@
 
 #include "types.hh"
 
-#include <sys/types.h>
+#ifndef _WIN32
+# include <sys/types.h>
+#endif
 
 namespace nix {
 
 std::string getUserName();
 
+#ifndef _WIN32
 /**
  * @return the given user's home directory from /etc/passwd.
  */
 Path getHomeOf(uid_t userId);
+#endif
 
 /**
  * @return $HOME or the user's home directory from /etc/passwd.
@@ -58,6 +62,8 @@ std::string expandTilde(std::string_view path);
 
 /**
  * Is the current user UID 0 on Unix?
+ *
+ * Currently always false on Windows, but that may change.
  */
 bool isRootUser();