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.cc

@@ -2,7 +2,12 @@
 #include "environment-variables.hh"
 #include "sync.hh"
 
-#include <sys/ioctl.h>
+#if _WIN32
+# include <io.h>
+# define isatty _isatty
+#else
+# include <sys/ioctl.h>
+#endif
 #include <unistd.h>
 
 namespace nix {
@@ -92,6 +97,7 @@ std::string filterANSIEscapes(std::string_view s, bool filterAll, unsigned int w
 static Sync<std::pair<unsigned short, unsigned short>> windowSize{{0, 0}};
 
 
+#ifndef _WIN32
 void updateWindowSize()
 {
     struct winsize ws;
@@ -101,6 +107,7 @@ void updateWindowSize()
         windowSize_->second = ws.ws_col;
     }
 }
+#endif
 
 
 std::pair<unsigned short, unsigned short> getWindowSize()