Add PTHREAD_STACK_MIN fallback to macOS builds

[mendeni]

PTHREAD_STACK_MIN is part of the POSIX threading standard, but is optional on macOS. Because this constant isn't defined, the compiler sees an undeclared identifier and throws an error.

This change adds a PTHREAD_STACK_MIN conditional to tdsh_posix.c so that macOS builds succeed.

[schikani]

Thanks for the fix, and welcome! It builds cleanly on Linux and all host tests pass.

One small request since you can test on macOS: POSIX defines PTHREAD_STACK_MIN in <limits.h>, which tdsh_posix.c never includes (on Linux it only arrives through <pthread.h>). Could you add #include <limits.h> with the other system headers and check whether macOS then defines it? If it does, it'll use the real value (16 KB on Apple Silicon, I believe), and your fallback can stay as a safety net for systems that still lack it.

I'll squash-merge once that's checked.

[mendeni]

@schikani I was able to build on macOS without fallback lines and only #include <limits.h>.

I left the fallback in place as a safety net as you described.

edit: fixed and typo

[schikani]

Thanks for checking on macOS and for the quick update! Merged.