This is an alternative firmware for the 858D+, it is based on madworms youyue-858D-plus repo, which is located here: https://github.com/madworm/Youyue-858D-plus
The main difference is, that this variant is pure C without all those Arduino specific things.
To compile the firmware on linux, you need at least
sudo apt-get install avr-libc gcc-avr make build-essential
All you need to do is running make in the directory where the Makefile is.
Upload using e.g. avr-dude, fuses are described here:
https://www.eevblog.com/forum/reviews/youyue-858d-some-reverse-engineering-custom-firmware/msg1320798/#msg1320798