Created attachment 209849 [details]
The authors' checks for BSD's pthread_set_name_np() fails, because the forget to include <pthread_np.h>.
The attached file can be dropped into the files/ subdirectory to fixe this trivial problem, enabling the functionality.
Can you submit upstream? I've tried to fix it in https://github.com/mpv-player/mpv/pull/6976 but have made a mistake batching other changes.