Download IrrlichtMt to `lib/irrlichtmt`, it will be used to satisfy the IrrlichtMt dependency that way:
- git clone --depth 1 https://github.com/minetest/irrlicht.git lib/irrlichtmt
+ git clone --branch 1.9.0mt7 --depth 1 https://github.com/minetest/irrlicht.git lib/irrlichtmt
Download source, without using Git: