As a heads-up to this thread: I have completed step 1 in the BUILD.TXT file, which results in a seemingly working version of 2.5.0 on my MacBook.
Next step will be to proceed with steps 2 and 3 and see if I can create a DMG.
I already found some points where the BUILD.TXT can be improved, I hope to be able to update that later (that will be my first time working with GitHub).
edit - steps 2 and 3 done too. I now have a working .DMG. The version is 2.5.0+95. I will still have to find out how to clone the 2.4.0 from Git instead of (what I assume to be) the most recent nightly build.