Issue #14359: Only use O_CLOEXEC in _posixmodule.c if it is defined.

Based on patch from Hervé Coatanhay.
diff --git a/Misc/ACKS b/Misc/ACKS
index 2b3dad5..77cec5c 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -173,6 +173,7 @@
 Andrew Clegg
 Brad Clements
 Steve Clift
+Hervé Coatanhay
 Nick Coghlan
 Josh Cogliati
 Dave Cole