build: stop defining _DEBUG, rely only on NDEBUG instead

NDEBUG is more standard on non-Microsoft platforms, and _DEBUG is
technically a reserved identifier.

Change-Id: I24c951032d50b37853dc1a38ac844b0be5ac8937
diff --git a/daemon/face/ethernet-protocol.cpp b/daemon/face/ethernet-protocol.cpp
index 24478f8..137ff27 100644
--- a/daemon/face/ethernet-protocol.cpp
+++ b/daemon/face/ethernet-protocol.cpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /*
- * Copyright (c) 2014-2022,  Regents of the University of California,
+ * Copyright (c) 2014-2024,  Regents of the University of California,
  *                           Arizona Board of Regents,
  *                           Colorado State University,
  *                           University Pierre & Marie Curie, Sorbonne University,
@@ -43,7 +43,7 @@
   if (ethertype != ETHERTYPE_NDN)
     return {nullptr, "Received frame with wrong ethertype: " + to_string(ethertype)};
 
-#ifdef _DEBUG
+#ifndef NDEBUG
   Address shost(eh->ether_shost);
   if (shost == localAddr)
     return {nullptr, "Received frame sent by this host"};