title: VS2017 'M_PI': undeclared identifier
body: <p>I'm trying to compile a library in C++ as dll windows with visual studio 2017.</p>
  
  <p>I have all C++/H compiled with clang 3.8 for android and it works.</p>
  
  <p>I created a new project as dynamic library with LLVM as platform toolset. Now I'm trying to build but there is an error :</p>
  
  <blockquote>
    <p>Error    C2065   'M_PI': undeclared identifier</p>
  </blockquote>
  
  <p>This error is present in many .cpp files. All linked .h contains :</p>
  
  <pre><code>#include &lt;cmath&gt;
  </code></pre>
  
  <p>I replaced by :</p>
  
  <pre><code>#define _USE_MATH_DEFINES // for C++  
  #include &lt;cmath&gt;
  </code></pre>
  
  <p>But it change nothing. What can I do ?</p>