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/libutil/windows/windows-error.cc

@@ -0,0 +1,31 @@
+#include "windows-error.hh"
+
+#include <error.h>
+#define WIN32_LEAN_AND_MEAN
+#include <windows.h>
+
+namespace nix {
+
+std::string WinError::renderError(DWORD lastError)
+{
+    LPSTR errorText = NULL;
+
+    FormatMessageA( FORMAT_MESSAGE_FROM_SYSTEM // use system message tables to retrieve error text
+                   |FORMAT_MESSAGE_ALLOCATE_BUFFER // allocate buffer on local heap for error text
+                   |FORMAT_MESSAGE_IGNORE_INSERTS,   // Important! will fail otherwise, since we're not  (and CANNOT) pass insertion parameters
+                    NULL,    // unused with FORMAT_MESSAGE_FROM_SYSTEM
+                    lastError,
+                    MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
+                    (LPTSTR)&errorText,  // output
+                    0, // minimum size for output buffer
+                    NULL);   // arguments - see note
+
+    if (NULL != errorText ) {
+        std::string s2 { errorText };
+        LocalFree(errorText);
+        return s2;
+    }
+    return fmt("CODE=%d", lastError);
+}
+
+}