>Pure make (you only see Makefile)
This is suspicious.
Wut?
Wut?
I tell macOS users to install VirtualBox and then build my software inside.
It is not nice on older as you can relatively easily make things work on most Unix systems if you don't use Linux specific calls.
Makefile-that-calls-3-other-build-systems isn't really using Make to build, is it? You are using Make as a glorified batch file. Could just as well use bash.
> I tell macOS users to install VirtualBox and then build my software inside.
This is pretty much the textbook definition of "non-portable build"