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/libmain/shared.cc

@@ -108,7 +108,9 @@ std::string getArg(const std::string & opt,
     return *i;
 }
 
+#ifndef _WIN32
 static void sigHandler(int signo) { }
+#endif
 
 
 void initNix()
@@ -121,6 +123,7 @@ void initNix()
 
     initLibStore();
 
+#ifndef _WIN32
     unix::startSignalHandlerThread();
 
     /* Reset SIGCHLD to its default. */
@@ -135,6 +138,7 @@ void initNix()
     /* Install a dummy SIGUSR1 handler for use with pthread_kill(). */
     act.sa_handler = sigHandler;
     if (sigaction(SIGUSR1, &act, 0)) throw SysError("handling SIGUSR1");
+#endif
 
 #if __APPLE__
     /* HACK: on darwin, we need can’t use sigprocmask with SIGWINCH.
@@ -156,21 +160,26 @@ void initNix()
     if (sigaction(SIGTRAP, &act, 0)) throw SysError("handling SIGTRAP");
 #endif
 
+#ifndef _WIN32
     /* Register a SIGSEGV handler to detect stack overflows.
        Why not initLibExpr()? initGC() is essentially that, but
        detectStackOverflow is not an instance of the init function concept, as
        it may have to be invoked more than once per process. */
     detectStackOverflow();
+#endif
 
     /* There is no privacy in the Nix system ;-)  At least not for
        now.  In particular, store objects should be readable by
        everybody. */
     umask(0022);
 
+#ifndef _WIN32
     /* Initialise the PRNG. */
     struct timeval tv;
     gettimeofday(&tv, 0);
     srandom(tv.tv_usec);
+#endif
+
 
 }
 
@@ -365,6 +374,9 @@ RunPager::RunPager()
     Pipe toPager;
     toPager.create();
 
+#ifdef _WIN32 // TODO re-enable on Windows, once we can start processes.
+    throw Error("Commit signature verification not implemented on Windows yet");
+#else
     pid = startProcess([&]() {
         if (dup2(toPager.readSide.get(), STDIN_FILENO) == -1)
             throw SysError("dupping stdin");
@@ -383,17 +395,20 @@ RunPager::RunPager()
     std_out = fcntl(STDOUT_FILENO, F_DUPFD_CLOEXEC, 0);
     if (dup2(toPager.writeSide.get(), STDOUT_FILENO) == -1)
         throw SysError("dupping standard output");
+#endif
 }
 
 
 RunPager::~RunPager()
 {
     try {
+#ifndef _WIN32 // TODO re-enable on Windows, once we can start processes.
         if (pid != -1) {
             std::cout.flush();
             dup2(std_out, STDOUT_FILENO);
             pid.wait();
         }
+#endif
     } catch (...) {
         ignoreException();
     }