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/build/hook-instance.hh

@@ -1,41 +0,0 @@
-#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();
-};
-
-}