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/lock.hh

@@ -0,0 +1,45 @@
+#pragma once
+///@file
+
+#include "types.hh"
+
+#include <optional>
+
+#include <sys/types.h>
+
+namespace nix {
+
+struct UserLock
+{
+    virtual ~UserLock() { }
+
+    /**
+     * Get the first and last UID.
+     */
+    std::pair<uid_t, uid_t> getUIDRange()
+    {
+        auto first = getUID();
+        return {first, first + getUIDCount() - 1};
+    }
+
+    /**
+     * Get the first UID.
+     */
+    virtual uid_t getUID() = 0;
+
+    virtual uid_t getUIDCount() = 0;
+
+    virtual gid_t getGID() = 0;
+
+    virtual std::vector<gid_t> getSupplementaryGIDs() = 0;
+};
+
+/**
+ * Acquire a user lock for a UID range of size `nrIds`. Note that this
+ * may return nullptr if no user is available.
+ */
+std::unique_ptr<UserLock> acquireUserLock(uid_t nrIds, bool useUserNamespace);
+
+bool useBuildUsers();
+
+}