What do you mean by portability? Are you able to confidently write a Makefile that works on any Linux distro, BSDs, MacOS and Windows? Or for you, portability

What do you mean by portability? Are you able to confidently write a Makefile that works on any Linux distro, BSDs, MacOS and Windows? Or for you, portability means Linux systems only? Honestly curious, because I've gone to the trouble of writing my own build system just so I can use the same build file on any OS whatsoever (which to me, means using a cross platform language for everything, not relying on bash or any other shell).