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/args.cc

@@ -9,7 +9,9 @@
 #include <fstream>
 #include <string>
 #include <regex>
-#include <glob.h>
+#ifndef _WIN32
+# include <glob.h>
+#endif
 
 namespace nix {
 
@@ -547,6 +549,7 @@ nlohmann::json Args::toJSON()
 static void _completePath(AddCompletions & completions, std::string_view prefix, bool onlyDirs)
 {
     completions.setType(Completions::Type::Filenames);
+    #ifndef _WIN32 // TODO implement globbing completions on Windows
     glob_t globbuf;
     int flags = GLOB_NOESCAPE;
     #ifdef GLOB_ONLYDIR
@@ -564,6 +567,7 @@ static void _completePath(AddCompletions & completions, std::string_view prefix,
         }
     }
     globfree(&globbuf);
+    #endif
 }
 
 void Args::completePath(AddCompletions & completions, size_t, std::string_view prefix)