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/libexpr/primops.cc

@@ -28,7 +28,10 @@
 #include <algorithm>
 #include <cstring>
 #include <regex>
-#include <dlfcn.h>
+
+#ifndef _WIN32
+# include <dlfcn.h>
+#endif
 
 #include <cmath>
 
@@ -331,6 +334,8 @@ static RegisterPrimOp primop_import({
     }
 });
 
+#ifndef _WIN32 // TODO implement via DLL loading on Windows
+
 /* Want reasonable symbol names, so extern C */
 /* !!! Should we pass the Pos or the file name too? */
 extern "C" typedef void (*ValueInitializer)(EvalState & state, Value & v);
@@ -403,6 +408,8 @@ void prim_exec(EvalState & state, const PosIdx pos, Value * * args, Value & v)
     }
 }
 
+#endif
+
 /* Return a string representing the type of the expression. */
 static void prim_typeOf(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
@@ -4593,6 +4600,7 @@ void EvalState::createBaseEnv()
         )",
     });
 
+#ifndef _WIN32 // TODO implement on Windows
     // Miscellaneous
     if (evalSettings.enableNativeCode) {
         addPrimOp({
@@ -4606,6 +4614,7 @@ void EvalState::createBaseEnv()
             .fun = prim_exec,
         });
     }
+#endif
 
     addPrimOp({
         .name = "__traceVerbose",