Yeah, I know. :-/ I live without them, but they are freaking hilarious!
I'm very curious to see what you end up porting to MacOS.
AFAIK, there is no real POSIX compatibility layer for MacOS anywhere. SIOW template helps with using basic console input and output, but you will quickly learn that Mac OS really is GUI-driven.
And on the note of GUIs, it's time to get familiar with ResEdit (or, indeed any other Macintosh resource editor). It... takes some getting used to. On the plus side is that you end up with essentially a GUI builder.
BTW: Thanks for asking this question. Here's to hoping that we can get some more people motivated to do some Macintosh development!
I don't think the MPW compiler was ever updated for C99 support, but, I may be wrong there.