build: add -DNDEBUG=1 to clang flags

Author: anonrig

node.gyp

@@ -462,6 +462,7 @@
         'NODE_ARCH="<(target_arch)"',
         'NODE_PLATFORM="<(OS)"',
         'NODE_WANT_INTERNALS=1',
+        'NDEBUG=1',
       ],
 
       'includes': [
@@ -808,6 +809,7 @@
         # Warn when using deprecated V8 APIs.
         'V8_DEPRECATION_WARNINGS=1',
         'NODE_OPENSSL_SYSTEM_CERT_PATH="<(openssl_system_ca_path)"',
+        'NDEBUG=1',
       ],
 
       # - "C4244: conversion from 'type1' to 'type2', possible loss of data"