patch 8.0.1709: some non-C89 code may slip through

Problem:    Some non-C89 code may slip through.
Solution:   Enforce C89 in configure. Fix detected problems. (James McCoy,
            closes #2735)
diff --git a/src/if_python3.c b/src/if_python3.c
index b885deb..4360fe4 100644
--- a/src/if_python3.c
+++ b/src/if_python3.c
@@ -815,7 +815,7 @@
 #endif
     if (Py_IsInitialized())
     {
-	// acquire lock before finalizing
+	/* acquire lock before finalizing */
 	PyGILState_Ensure();
 
 	Py_Finalize();