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/libstore/store-api.cc

@@ -13,7 +13,6 @@
 #include "archive.hh"
 #include "callback.hh"
 #include "git.hh"
-#include "remote-store.hh"
 #include "posix-source-accessor.hh"
 // FIXME this should not be here, see TODO below on
 // `addMultipleToStore`.
@@ -21,6 +20,10 @@
 #include "signals.hh"
 #include "users.hh"
 
+#ifndef _WIN32
+# include "remote-store.hh"
+#endif
+
 #include <nlohmann/json.hpp>
 #include <regex>
 
@@ -1266,9 +1269,10 @@ Derivation Store::readInvalidDerivation(const StorePath & drvPath)
 
 }
 
-
-#include "local-store.hh"
-#include "uds-remote-store.hh"
+#ifndef _WIN32
+# include "local-store.hh"
+# include "uds-remote-store.hh"
+#endif
 
 
 namespace nix {
@@ -1286,6 +1290,9 @@ std::pair<std::string, Store::Params> splitUriAndParams(const std::string & uri_
     return {uri, params};
 }
 
+#ifdef _WIN32 // Unused on Windows because the next `#ifndef`
+[[maybe_unused]]
+#endif
 static bool isNonUriPath(const std::string & spec)
 {
     return
@@ -1298,6 +1305,9 @@ static bool isNonUriPath(const std::string & spec)
 
 std::shared_ptr<Store> openFromNonUri(const std::string & uri, const Store::Params & params)
 {
+    // TODO reenable on Windows once we have `LocalStore` and
+    // `UDSRemoteStore`.
+    #ifndef _WIN32
     if (uri == "" || uri == "auto") {
         auto stateDir = getOr(params, "state", settings.nixStateDir);
         if (access(stateDir.c_str(), R_OK | W_OK) == 0)
@@ -1342,6 +1352,9 @@ std::shared_ptr<Store> openFromNonUri(const std::string & uri, const Store::Para
     } else {
         return nullptr;
     }
+    #else
+    return nullptr;
+    #endif
 }
 
 // The `parseURL` function supports both IPv6 URIs as defined in