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/child.hh

@@ -0,0 +1,11 @@
+#pragma once
+///@file
+
+namespace nix {
+
+/**
+ * Common initialisation performed in child processes.
+ */
+void commonChildInit();
+
+}