The classic 68k Mac binary model is pretty far from what LLVM and GCC support even if they still have backends capable of compiling to older 68k CPUs (I'm not sure if that's case). Executable code is stored in the resource fork; the Mac SDKs require a compiler with support for a special syntax for trap-table based calls; the classic Mac Toolbox expects Pascal-format strings for most things.
helfire mentions Retro68; if you look at its README, you can get an idea of the scope of modifications required.
helfire mentions Retro68; if you look at its README, you can get an idea of the scope of modifications required.