[master] 0bbd1d06a build: Use -Wunused-but-set-variable if available

[Dridi Boukelmoune]

commit 0bbd1d06a05e83f311ab4946f8284a509e300051
Author: Dridi Boukelmoune <dridi.boukelmoune at gmail.com>
Date:   Wed Sep 22 18:43:57 2021 +0200

    build: Use -Wunused-but-set-variable if available
    
    Refs 02ada45c5d51

diff --git a/wflags.py b/wflags.py
index 9e9e4e902..1b8c48fc0 100644
--- a/wflags.py
+++ b/wflags.py
@@ -64,6 +64,7 @@ DESIRABLE_WFLAGS = [
     "-Wunused-parameter",
     "-Wunused-parameters",
     "-Wunused-result",
+    "-Wunused-but-set-variable",
     "-Wwrite-strings",
 ]