Fix typos in macro names
diff --git a/library/threading.c b/library/threading.c
index fa745af..5d48516 100644
--- a/library/threading.c
+++ b/library/threading.c
@@ -105,6 +105,6 @@
 
     return( 0 );
 }
-#endif /* POLARSSL_THREADING_ALT_C */
+#endif /* POLARSSL_THREADING_ALT */
 
 #endif /* POLARSSL_THREADING_C */