It doesn't compile without including i_threads.h, even though macOS does not have an implementation for threads in this version.
SRB2Classic/src/i_threads.c:316:39: error: unknown type name 'I_thread_fn'
316 | void I_spawn_thread(const char *name, I_thread_fn entry, void *userdata)
| ^
SRB2Classic/src/i_threads.c:334:19: error: unknown type name 'I_mutex'
334 | void I_lock_mutex(I_mutex *anchor)
| ^
SRB2Classic/src/i_threads.c:339:21: error: unknown type name 'I_mutex'
339 | void I_unlock_mutex(I_mutex id)
| ^
SRB2Classic/src/i_threads.c:344:18: error: unknown type name 'I_cond'
344 | void I_hold_cond(I_cond *cond_anchor, I_mutex mutex_id)
| ^
SRB2Classic/src/i_threads.c:344:39: error: unknown type name 'I_mutex'
344 | void I_hold_cond(I_cond *cond_anchor, I_mutex mutex_id)
| ^
SRB2Classic/src/i_threads.c:350:22: error: unknown type name 'I_cond'
350 | void I_wake_one_cond(I_cond *anchor)
| ^
SRB2Classic/src/i_threads.c:355:22: error: unknown type name 'I_cond'
355 | void I_wake_all_cond(I_cond *anchor)
| ^