title: Why can't I compile IsNan in C++ Builder
body: <p>I'm using C++ Builder XE and want to check a float value is valid. According to the help, in math.h, there's a call </p>
  
  <pre><code>bool IsNan(float value)
  </code></pre>
  
  <p>However, when I try to compile this, it tells me</p>
  
  <pre><code>Call to undefined function 'IsNan'
  </code></pre>
  
  <p>Looking in math.h, there's no such thing as isnan (I used a non-case sensitive search)</p>
  
  <p>Am I doing something silly, or what ?</p>