Hi nonfinite,
I have never try to build it for mac osx.
But I use sdl 1.2 and rtaudio and those lib support it.
The error displayed come from the lack of alsa on osx.
Osx use something like coreaudio.
So you should change the : -D__LINUX_ALSA__ with
-DMACOSX_CORE for the RtAudio, so you just have to modify this and continue to use the debian kind of makefile, like you do.
Then you will need the header file and "lib" for sdl 1.2 :
https://www.libsdl.org/download-1.2.php
Put it on the directory you are using, I can't tell you the good path of this but you will have to play a bit to help the complier and linker to find the .h and the lib file.
Rtaudio doc is here :
https://www.music.mcgill.ca/~gary/rtaudio/
The -Dsomestuff is described on RtAudio.h or cpp
Tell me if it works, it should but it is not really easy if you don't use a compiler every day.
Johann