title: CFLAGS not working on Windows when compiling with node-gyp
body: <p>For some reason when I compile using node-gyp on Windows, CFLAGS are ignored. Does anyone maybe know the reason? This is my code:</p>
  
  <p>Binding.gyp</p>
  
  <pre><code>{
    "targets": [
      {
        "target_name": "helloWindows",
        "sources": [ "helloWindows.cpp" ],
        "cflags": [ "-D_MY_FLAG"],
      }
    ]
  }
  </code></pre>
  
  <p>helloWindows.cpp</p>
  
  <pre><code>using namespace v8;
  
  #if defined(_MY_FLAG) 
  void SuperFunction(const v8::FunctionCallbackInfo&lt;Value&gt;&amp; args) {
    Isolate* isolate = Isolate::GetCurrent();
    HandleScope scope(isolate);
  
    args.GetReturnValue().Set(String::NewFromUtf8(isolate, "Hello Antirreni!"));
  
  }
  
  void init(Handle&lt;Object&gt; target) {
      NODE_SET_METHOD(target, "hello", SuperFunction);
  }
  
  NODE_MODULE(helloWindows, init);
  
  #endif
  </code></pre>
  
  <p>Thanks in advance :)</p>