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"