> only available since GNU make 4.0, and some Linux distributions still have older make, made it a hassle to setup. This should improve over time, of course.
That feels like an outdated statement, make 4.0 came out in 2013. The only non-EOL system I can think of off the top of my head that isn't shipping >=4.0 is macOS (because they won't upgrade to anything GPLv3).
However, unfortunately there's a popular CI system (cough CircleCI cough) that's still using Ubuntu 14.04 by default, which has been EOL for almost a year now. (And others, such as Travis-CI, used it by default right up until it was EOL).
Oh, if you appreciate gmsl, you might also appreciate a book by the same author ('jgrahamc), The GNU Make Book. Some of the things in gmsl are certainly hacks, but a lot of it is quite reasonable if you start reading Make macros as "lisp, but with a bunch of extra dollar-signs and commas".