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/windows/build.cc

@@ -0,0 +1,37 @@
+#include "store-api.hh"
+#include "build-result.hh"
+
+namespace nix {
+
+void Store::buildPaths(const std::vector<DerivedPath> & reqs, BuildMode buildMode, std::shared_ptr<Store> evalStore)
+{
+    unsupported("buildPaths");
+}
+
+std::vector<KeyedBuildResult> Store::buildPathsWithResults(
+    const std::vector<DerivedPath> & reqs,
+    BuildMode buildMode,
+    std::shared_ptr<Store> evalStore)
+{
+    unsupported("buildPathsWithResults");
+}
+
+BuildResult Store::buildDerivation(const StorePath & drvPath, const BasicDerivation & drv,
+    BuildMode buildMode)
+{
+    unsupported("buildDerivation");
+}
+
+
+void Store::ensurePath(const StorePath & path)
+{
+    unsupported("ensurePath");
+}
+
+
+void Store::repairPath(const StorePath & path)
+{
+    unsupported("repairPath");
+}
+
+}