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/file-descriptor.cc

@@ -5,6 +5,11 @@
 
 #include <fcntl.h>
 #include <unistd.h>
+#ifdef _WIN32
+# include <winnt.h>
+# include <fileapi.h>
+# include "windows-error.hh"
+#endif
 
 namespace nix {
 
@@ -20,7 +25,13 @@ std::string drainFD(Descriptor fd, bool block, const size_t reserveSize)
     // the parser needs two extra bytes to append terminating characters, other users will
     // not care very much about the extra memory.
     StringSink sink(reserveSize + 2);
+#ifdef _WIN32
+    // non-blocking is not supported this way on Windows
+    assert(block);
+    drainFD(fd, sink);
+#else
     drainFD(fd, sink, block);
+#endif
     return std::move(sink.s);
 }
 
@@ -68,9 +79,15 @@ Descriptor AutoCloseFD::get() const
 void AutoCloseFD::close()
 {
     if (fd != INVALID_DESCRIPTOR) {
-        if(::close(fd) == -1)
+        if(
+#ifdef _WIN32
+           ::CloseHandle(fd)
+#else
+           ::close(fd)
+#endif
+           == -1)
             /* This should never happen. */
-            throw SysError("closing file descriptor %1%", fd);
+            throw NativeSysError("closing file descriptor %1%", fd);
         fd = INVALID_DESCRIPTOR;
     }
 }
@@ -80,14 +97,16 @@ void AutoCloseFD::fsync()
     if (fd != INVALID_DESCRIPTOR) {
         int result;
         result =
-#if __APPLE__
+#ifdef _WIN32
+            ::FlushFileBuffers(fd)
+#elif __APPLE__
             ::fcntl(fd, F_FULLFSYNC)
 #else
             ::fsync(fd)
 #endif
             ;
         if (result == -1)
-            throw SysError("fsync file descriptor %1%", fd);
+            throw NativeSysError("fsync file descriptor %1%", fd);
     }
 }