I never changed textures, so I had to take a look.
I assume you know how to compile (with Celeron55's readme.txt
https://github.com/celeron55/minetest ), and that you've done that.
I found the following folder:
---> /home/"Topywo"/celeron55-minetest-4b2cc38/textures/all
In it is a "textures_here.txt" file.
The text inside it says: If you haven't modified the texture_path setting, you can copy textures of your texture packs into here. Folders are currently not supported. So my guess would be to place texture pack .png 's in here.
Hope this helps you out.