LWIP_PLATFORM_DIAG: check if LWIP_PLATFORM_DIAG is defined as needed.

-- LWIP_STATS and LWIP_DEBUG require this macro

src/core/stats.c

@@ -40,6 +40,10 @@
 
 #if LWIP_STATS /* don't build if not configured for use in lwipopts.h */
 
+#ifndef LWIP_PLATFORM_DIAG
+  #error "If you want to use LWIP_STATS, LWIP_PLATFORM_DIAG needs to be defined in your arch/cc.h"
+#endif
+
 #include "lwip/def.h"
 #include "lwip/stats.h"
 #include "lwip/mem.h"

src/include/lwip/debug.h

@@ -83,6 +83,9 @@
 #endif /* LWIP_ERROR */
 
 #ifdef LWIP_DEBUG
+#ifndef LWIP_PLATFORM_DIAG
+  #error "If you want to use LWIP_DEBUG, LWIP_PLATFORM_DIAG needs to be defined in your arch/cc.h"
+#endif
 /** print debug message only if debug message type is enabled...
  *  AND is of correct type AND is at least LWIP_DBG_LEVEL
  */