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/unix/build/hook-instance.hh

@@ -0,0 +1,41 @@
+#pragma once
+///@file
+
+#include "logging.hh"
+#include "serialise.hh"
+#include "processes.hh"
+
+namespace nix {
+
+struct HookInstance
+{
+    /**
+     * Pipes for talking to the build hook.
+     */
+    Pipe toHook;
+
+    /**
+     * Pipe for the hook's standard output/error.
+     */
+    Pipe fromHook;
+
+    /**
+     * Pipe for the builder's standard output/error.
+     */
+    Pipe builderOut;
+
+    /**
+     * The process ID of the hook.
+     */
+    Pid pid;
+
+    FdSink sink;
+
+    std::map<ActivityId, Activity> activities;
+
+    HookInstance();
+
+    ~HookInstance();
+};
+
+}