The usage of threads within libraries is an internal matter, so I think
Emacs should be prepared for threads always when<pthread.h> is available.
That's also my impression.
Are you thinking of sigwait? Other than sigwait, there is no way to "set up
our signal handlers so we don't need this", where this ==
SIGNAL_THREAD_CHECK.
I was afraid so.
SIGNAL_THREAD_CHECK is not unreliable. It does not drop signals delivered
to the wrong thread, it forwards them to the main thread.
Indeed, I had misread the code.
Stefan