title: Calculate atan2 without std functions or C99
body: <p>I am calculating angles from a 3-axis accelerometer, but my compiler doesn't have a atan or atan2 function. It has a reserved memory slot, but it calls a function i can't find in any files.</p>
  
  <p>My compiler is Keil µVision 4 running the ARMCC compiler.
  The compiles has the file math.h, but the function is extern and doesn't exist: </p>
  
  <blockquote>
  <pre><code>  extern _ARMABI double atan2(double /*y*/, double /*x*/);
  </code></pre>
  </blockquote>
  
  <p>Is there a lib or function I can include that has the function arctan implemented? 
  Or is there an alternative function to calculate angles from accelerometer? I need full 3-axis calibration of the angles.</p>
  
  <p>Edit: I was hoping to avoid a table full of pre-calculated values.</p>