Well look like its compiling some sort of *nix library..
It checks weather uname returns AmigaOS or Darwin.. and then compiles accordingly.
Darwin is MAC OS as you probably know which is a fork from FreeBSD.. *I believe*
I would guess since Mac OSX X86/PPC is supported then the AmigaOS probably means OS 4.x PPC since thats like the Mac PPC and Im guessing OS 4.x probably has more *nix feel to it and still reports under uname as being AmigaOS?.. I've never used it so dont 100% know..