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/filetransfer.cc

@@ -516,10 +516,12 @@ struct curlFileTransfer : public FileTransfer
 
     Sync<State> state_;
 
+    #ifndef _WIN32 // TODO need graceful async exit support on Windows?
     /* We can't use a std::condition_variable to wake up the curl
        thread, because it only monitors file descriptors. So use a
        pipe instead. */
     Pipe wakeupPipe;
+    #endif
 
     std::thread workerThread;
 
@@ -539,8 +541,10 @@ struct curlFileTransfer : public FileTransfer
             fileTransferSettings.httpConnections.get());
         #endif
 
+        #ifndef _WIN32 // TODO need graceful async exit support on Windows?
         wakeupPipe.create();
         fcntl(wakeupPipe.readSide.get(), F_SETFL, O_NONBLOCK);
+        #endif
 
         workerThread = std::thread([&]() { workerThreadEntry(); });
     }
@@ -561,15 +565,19 @@ struct curlFileTransfer : public FileTransfer
             auto state(state_.lock());
             state->quit = true;
         }
+        #ifndef _WIN32 // TODO need graceful async exit support on Windows?
         writeFull(wakeupPipe.writeSide.get(), " ", false);
+        #endif
     }
 
     void workerThreadMain()
     {
         /* Cause this thread to be notified on SIGINT. */
+        #ifndef _WIN32 // TODO need graceful async exit support on Windows?
         auto callback = createInterruptCallback([&]() {
             stopWorkerThread();
         });
+        #endif
 
         #if __linux__
         unshareFilesystem();
@@ -607,9 +615,11 @@ struct curlFileTransfer : public FileTransfer
             /* Wait for activity, including wakeup events. */
             int numfds = 0;
             struct curl_waitfd extraFDs[1];
+            #ifndef _WIN32 // TODO need graceful async exit support on Windows?
             extraFDs[0].fd = wakeupPipe.readSide.get();
             extraFDs[0].events = CURL_WAIT_POLLIN;
             extraFDs[0].revents = 0;
+            #endif
             long maxSleepTimeMs = items.empty() ? 10000 : 100;
             auto sleepTimeMs =
                 nextWakeup != std::chrono::steady_clock::time_point()
@@ -693,7 +703,9 @@ struct curlFileTransfer : public FileTransfer
                 throw nix::Error("cannot enqueue download request because the download thread is shutting down");
             state->incoming.push(item);
         }
+        #ifndef _WIN32 // TODO need graceful async exit support on Windows?
         writeFull(wakeupPipe.writeSide.get(), " ");
+        #endif
     }
 
 #if ENABLE_S3