build: stop defining _DEBUG, rely on NDEBUG instead

NDEBUG is more standard on non-Microsoft platforms

Change-Id: I6faec848b64abfc291b85903106fc48450943f9a
diff --git a/.waf-tools/default-compiler-flags.py b/.waf-tools/default-compiler-flags.py
index 2941fe1..6869097 100644
--- a/.waf-tools/default-compiler-flags.py
+++ b/.waf-tools/default-compiler-flags.py
@@ -136,7 +136,7 @@
 
     def getDebugFlags(self, conf):
         """Get dict of CXXFLAGS, LINKFLAGS, and DEFINES that are needed only in debug mode"""
-        return {'CXXFLAGS': [], 'LINKFLAGS': [], 'DEFINES': ['_DEBUG']}
+        return {'CXXFLAGS': [], 'LINKFLAGS': [], 'DEFINES': []}
 
     def getOptimizedFlags(self, conf):
         """Get dict of CXXFLAGS, LINKFLAGS, and DEFINES that are needed only in optimized mode"""