That's pretty neat! But is there a link to the “future POSIX standard”? A few months ago I tried looking for it on https://pubs.opengroup.org/, but couldn't.
To answer my own question, if this[1] StackExchange to be believed, you need to join the mailing list of something called “The Austin Group”. Although some fairly informative meeting minutes are available publicly here[2].
> As the behavior described in the desired action hasn't actually been implemented, to our knowledge, and there is insufficient consensus amongst the variants that have been implemented to pick one as the de facto standard this request is being rejected. Should the industry come to a consensus a new bug reflecting that should be filed.
TL;DR: no way forward for adding if/else statements to make for now.
To be fair, if you need conditionals in target bodies, you're probably better off putting the body into a separate file as a shell script. And if you need conditionals in macros, there's the old macro-in-macro approach:
[1]: https://unix.stackexchange.com/q/607333/105635
[2]: https://www.opengroup.org/austin/docreg.html