Do not define _POSIX_THREADS if unistd.h defines it.
Check for pthread_sigmask before using it. Fixes remaining problem in #470781.
diff --git a/Python/thread_pthread.h b/Python/thread_pthread.h
index 2592d39..7170c8a 100644
--- a/Python/thread_pthread.h
+++ b/Python/thread_pthread.h
@@ -75,7 +75,7 @@
* other UNIX International compliant systems that don't have the full
* pthread implementation.
*/
-#ifdef PY_PTHREAD_STD
+#ifdef HAVE_PTHREAD_SIGMASK
# define SET_THREAD_SIGMASK pthread_sigmask
#else
# define SET_THREAD_SIGMASK sigprocmask