How to set the path for userpref.blend file?

After compiling Blender as a Python module I realized that userpref.blend does not exist in ~/.config/blender/2.79/config. I want to use a userpref.blend file that I have on my desktop computer (when using Blender with the GUI). So I wonder how can one manually set the path to userpref.blend file using Blender's Python API?

Setting the environmental variable BLENDER_USER_CONFIG would do the job. So Blender assumes the path to the userpref.blend file would be BLENDER_USER_CONFIG/userpref.blend