Issue #16537: Check whether self.extensions is empty in setup.py.

Patch by Jonathan Hosmer.
diff --git a/Misc/ACKS b/Misc/ACKS
index 1821682..f077ff7 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -588,6 +588,7 @@
 Nadav Horesh
 Alon Horev
 Jan Hosang
+Jonathan Hosmer
 Alan Hourihane
 Ken Howard
 Brad Howes