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/ssh.hh

@@ -21,7 +21,9 @@ private:
 
     struct State
     {
+#ifndef _WIN32 // TODO re-enable on Windows, once we can start processes.
         Pid sshMaster;
+#endif
         std::unique_ptr<AutoDelete> tmpDir;
         Path socketPath;
     };
@@ -31,13 +33,19 @@ private:
     void addCommonSSHOpts(Strings & args);
     bool isMasterRunning();
 
+#ifndef _WIN32 // TODO re-enable on Windows, once we can start processes.
+    Path startMaster();
+#endif
+
 public:
 
     SSHMaster(const std::string & host, const std::string & keyFile, const std::string & sshPublicHostKey, bool useMaster, bool compress, int logFD = -1);
 
     struct Connection
     {
+#ifndef _WIN32 // TODO re-enable on Windows, once we can start processes.
         Pid sshPid;
+#endif
         AutoCloseFD out, in;
     };
 
@@ -51,8 +59,6 @@ public:
     std::unique_ptr<Connection> startCommand(
         Strings && command,
         Strings && extraSshArgs = {});
-
-    Path startMaster();
 };
 
 }