--- Python-3.9.5/Modules/posixmodule.h	2021-02-21 20:22:43.839024851 +0100
+++ Python-3.9.5/Modules/posixmodule.h	2021-02-21 20:28:35.823057995 +0100
@@ -19,7 +19,8 @@
 #endif /* MS_WINDOWS */
 
 #if defined(PYPTHREAD_SIGMASK) || defined(HAVE_SIGWAIT) || \
-        defined(HAVE_SIGWAITINFO) || defined(HAVE_SIGTIMEDWAIT)
+        defined(HAVE_SIGWAITINFO) || defined(HAVE_SIGTIMEDWAIT) || \
+        defined(__serenity__)
 # define HAVE_SIGSET_T
 #endif