This is chalk full of bad advice like "use GNU make and bash" that it's obvious to me it was written by someone who only knows GNU/Linux and doesn't know UNIX (or much else). Blind leading the blind again. Don't write GNU-specific constructs unless you have no other choice; use ksh instead of bash, it's portable and a standard and has been long before the GNU/Linux abomination. What nonsense I get to read...
If somebody is targeting inter-Unix builds, Autotools is probably a better pick than hand-rolling a Makefile.
But realistically, Linux has captured 75% or more of the entire *nix market. A lot of developers spend their entire careers in Linux these days, and that trend isn't likely to swing back towards other Unix flavors anytime soon.
As long as a developer understands what users will be using their Makefile, it shouldn't matter if they target platform-specific features. The BSD ports tree uses a bunch of BSD Make features that aren't in GNU Make. And that's totally fine, because they know their target users.
Like let's say I'm writing a Makefile that wraps tools which don't run on anything other than Linux. Why would I care about portability across Make implementations?
If a dev doesn't know their users' environments (other than being POSIXy), probably they shouldn't be writing raw Make of any flavor. And if they do know their users, then it isn't a big deal to rely on some platform-specific features.
