What does posix compliant* even mean for make? The posix standard doesn't even require support for c++! That's a VERY portable Makefile. It would work on any make from the late seventies (assuming you also had a c++ compiler and system Makefiles that knew how to use it). It only uses a small subset of the rules in the standard.

* https://pubs.opengroup.org/onlinepubs/009695399/utilities/ma...