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/logging.cc

@@ -116,7 +116,13 @@ Verbosity verbosity = lvlInfo;
 void writeToStderr(std::string_view s)
 {
     try {
-        writeFull(STDERR_FILENO, s, false);
+        writeFull(
+#ifdef _WIN32
+            GetStdHandle(STD_ERROR_HANDLE),
+#else
+            STDERR_FILENO,
+#endif
+            s, false);
     } catch (SystemError & e) {
         /* Ignore failing writes to stderr.  We need to ignore write
            errors to ensure that cleanup code that logs to stderr runs
@@ -132,9 +138,18 @@ Logger * makeSimpleLogger(bool printBuildLogs)
 
 std::atomic<uint64_t> nextId{0};
 
+static uint64_t getPid()
+{
+#ifndef _WIN32
+    return getpid();
+#else
+    return GetCurrentProcessId();
+#endif
+}
+
 Activity::Activity(Logger & logger, Verbosity lvl, ActivityType type,
     const std::string & s, const Logger::Fields & fields, ActivityId parent)
-    : logger(logger), id(nextId++ + (((uint64_t) getpid()) << 32))
+    : logger(logger), id(nextId++ + (((uint64_t) getPid()) << 32))
 {
     logger.startActivity(id, lvl, type, s, fields, parent);
 }