Builds on macOS

[jminor]

This PR allows Sassy to build and run on macOS, but sadly it crashes at runtime in the JIT-compiled eval_all() during audio callback.

I suspect this is the same issue mentioned in #1 on Linux.

Any clues about how to diagnose the problem?

Note that the macOS build settings in the Makefile are slightly odd given that I'm building on an M1 arm64 machine, but asking the compiler to build an x86_64 executable to run under Rosetta 2. In theory this should work, even with JIT code, as documented here: https://developer.apple.com/documentation/apple-silicon/about-the-rosetta-translation-environment but if someone has an Intel Mac to test on, it would be good to verify that this is/isn't related to the crash.

[jminor]

Here's a screenshot of the macOS build running.

Note that formulas with arithmetic work 2*99+17+(15*9) but if you try to call any function time() or mention another cell a4 then it crashes inside the audio callback, inside eval_all

Curiously the debugger seems to have lost its place within eval_all which might imply some stack corruption?

Thread 7 Crashed:: SDLAudioP2
0   <translation info unavailable>	       0x100c084ec ???
1   sassy                         	       0x100449a62 eval_all() + 162
2   sassy                         	       0x100449a62 eval_all() + 162
3   sassy                         	       0x100449caf sdl2_audiomixer(void*, unsigned char*, int) + 415
4   libSDL2-2.0.0.dylib           	       0x10929884f SDL_RunAudio + 208
5   libSDL2-2.0.0.dylib           	       0x1092ee767 SDL_RunThread + 53
6   libSDL2-2.0.0.dylib           	       0x109356385 RunThread + 9
7   libsystem_pthread.dylib       	    0x7ff8124bb259 _pthread_start + 125
8   libsystem_pthread.dylib       	    0x7ff8124b6c7b thread_start + 15