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/libcmd/repl-interacter.cc

@@ -137,6 +137,7 @@ static constexpr const char * promptForType(ReplPromptType promptType)
 
 bool ReadlineLikeInteracter::getLine(std::string & input, ReplPromptType promptType)
 {
+#ifndef _WIN32 // TODO use more signals.hh for this
     struct sigaction act, old;
     sigset_t savedSignalMask, set;
 
@@ -161,9 +162,12 @@ bool ReadlineLikeInteracter::getLine(std::string & input, ReplPromptType promptT
     };
 
     setupSignals();
+#endif
     char * s = readline(promptForType(promptType));
     Finally doFree([&]() { free(s); });
+#ifndef _WIN32 // TODO use more signals.hh for this
     restoreSignals();
+#endif
 
     if (g_signal_received) {
         g_signal_received = 0;