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/processes.cc

@@ -0,0 +1,48 @@
+#include "current-process.hh"
+#include "environment-variables.hh"
+#include "signals.hh"
+#include "processes.hh"
+#include "finally.hh"
+#include "serialise.hh"
+
+#include <cerrno>
+#include <cstdlib>
+#include <cstring>
+#include <future>
+#include <iostream>
+#include <sstream>
+#include <thread>
+
+#include <sys/types.h>
+#include <unistd.h>
+
+#ifdef __APPLE__
+# include <sys/syscall.h>
+#endif
+
+#ifdef __linux__
+# include <sys/prctl.h>
+# include <sys/mman.h>
+#endif
+
+
+namespace nix {
+
+std::string runProgram(Path program, bool searchPath, const Strings & args,
+    const std::optional<std::string> & input, bool isInteractive)
+{
+    throw UnimplementedError("Cannot shell out to git on Windows yet");
+}
+
+// Output = error code + "standard out" output stream
+std::pair<int, std::string> runProgram(RunOptions && options)
+{
+    throw UnimplementedError("Cannot shell out to git on Windows yet");
+}
+
+void runProgram2(const RunOptions & options)
+{
+    throw UnimplementedError("Cannot shell out to git on Windows yet");
+}
+
+}