Mostly nothing, but writing a makefile and only evaluating against it makes it difficult to guarantee that you will see the same behaviour on a non-GNU system (which has become more relevant these days as even systems based on the Linux kernel are no longer guaranteed to run a GNU userland). What makes matters worse is that even if you grab a BSD make implementation (there are
several), there are no guarantees there either, although the number of non-POSIX extensions are far far fewer. Thus you are left to committing things to memory and it is not easy to get it right.
With a strict POSIX implementation, you could develop against it and if it works correctly on your system it should also work correctly with pretty much any other make implementation out there.
Personally, I was so frustrated recently about this that I was looking for exactly what pdpmake offers and could not find anything. Thus I am now very thankful that I do not need to write it myself. But there is still plenty of room for strict POSIX tools like this for: awk, sed, m4, etc.