This Darwin-specific target builds each stb library with macOS-specific
options so that the results are universal static libraries that should
work on Intel (x86_64) and Apple Silicon (arm64) machines. They also
should work on macOS 10.12 and above (which should match what Odin
compiles for).
The default Makefile target will build the darwin rule if its invoked on
a macOS machine, otherwise it'll invoke the more general unix target.
