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/libstore/daemon.cc

@@ -1,5 +1,4 @@
 #include "daemon.hh"
-#include "monitor-fd.hh"
 #include "signals.hh"
 #include "worker-protocol.hh"
 #include "worker-protocol-impl.hh"
@@ -16,6 +15,10 @@
 #include "args.hh"
 #include "git.hh"
 
+#ifndef _WIN32 // TODO need graceful async exit support on Windows?
+# include "monitor-fd.hh"
+#endif
+
 namespace nix::daemon {
 
 Sink & operator << (Sink & sink, const Logger::Fields & fields)
@@ -1018,7 +1021,9 @@ void processConnection(
     TrustedFlag trusted,
     RecursiveFlag recursive)
 {
+#ifndef _WIN32 // TODO need graceful async exit support on Windows?
     auto monitor = !recursive ? std::make_unique<MonitorFdHup>(from.fd) : nullptr;
+#endif
 
     /* Exchange the greeting. */
     unsigned int magic = readInt(from);