Fix MinGW issues, take 2.

format.cc

@@ -301,7 +301,12 @@ int fmt::internal::StrError(
   if (message == buffer && strlen(buffer) == buffer_size - 1)
     result = ERANGE;
   buffer = message;
-#elif defined(_WIN32) && !defined(__MINGW32__)
+#elif __MINGW32__
+  errno = 0;
+  (void)buffer_size;
+  buffer = strerror(error_code);
+  result = errno;
+#elif _WIN32
   result = strerror_s(buffer, buffer_size, error_code);
   // If the buffer is full then the message is probably truncated.
   if (result == 0 && std::strlen(buffer) == buffer_size - 1)