Hi, I managed to get this compiling on Linux on 4.15 with a few changes, I hope these help someone!
Diff from 4.15 src files from gumroad:
https://github.com/faemir/misc/blob/master/customgrav4.15linuxfix
Mhousse1247 - I don’t know if these might help you too, and hope you don’t mind me posting this.