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/terminal.hh

@@ -21,12 +21,16 @@ std::string filterANSIEscapes(std::string_view s,
     bool filterAll = false,
     unsigned int width = std::numeric_limits<unsigned int>::max());
 
+#ifndef _WIN32
+
 /**
  * Recalculate the window size, updating a global variable. Used in the
  * `SIGWINCH` signal handler.
  */
 void updateWindowSize();
 
+#endif
+
 /**
  * @return the number of rows and columns of the terminal.
  *