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/libexpr/search-path.hh

@@ -8,6 +8,9 @@
 
 namespace nix {
 
+// Do not want the windows macro (alias to `SearchPathA`)
+#undef SearchPath
+
 /**
  * A "search path" is a list of ways look for something, used with
  * `builtins.findFile` and `< >` lookup expressions.