Unfortunately, neither the mutex functions nor pthread_kill() are listed as async-signal-safe:
http://www.opengroup.org/onlinepubs/009695399/functions/xsh_chap02_04.html

Personally, I'd be just as happy to raise an exception if an attempt is made to import both signal and threading: doing it safely and reliably is just too difficult, so we shouldn't promote a false sense of security.