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/tests/other/face-benchmark.cpp b/tests/other/face-benchmark.cpp
index c5c666d..4c1ac6a 100644
--- a/tests/other/face-benchmark.cpp
+++ b/tests/other/face-benchmark.cpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /*
- * Copyright (c) 2014-2023,  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,
@@ -177,7 +177,7 @@
 int
 main(int argc, char** argv)
 {
-#ifdef _DEBUG
+#ifndef NDEBUG
   std::cerr << "Benchmark compiled in debug mode is unreliable, please compile in release mode.\n";
 #endif