Fixing compiler warnings

Adjusting toolchain files which compiler warnings to enable.

Fixing compiler warnings reported by Arm Clang and Arm GCC.

Change-Id: I715e875904ffd7ecfe994d3093cdf066373981b1
diff --git a/applications/message_handler/main.cpp b/applications/message_handler/main.cpp
index 9f7c7cc..9b36f84 100644
--- a/applications/message_handler/main.cpp
+++ b/applications/message_handler/main.cpp
@@ -128,9 +128,11 @@
 
 namespace {
 
+#ifdef MHU_IRQ
 void mailboxIrqHandler() {
     mailbox.handleMessage();
 }
+#endif
 
 void inferenceTask(void *pvParameters) {
     printf("Starting inference task\n");