title: How to check if bool already exists
body: <p>I an developing a lib for use on an embedded platform. I have code in a header that is part of the lib with 
  <code>typdef enum bool {false, true} bool;</code></p>
  
  <p>If the lib user has already defined a type named bool, how can I code this so that my lib does not attempt to re-declare it?</p>
  
  <p>Currently I have used #defines</p>
  
  <pre><code>#ifndef _BOOL 
  #define _BOOL 
  typedef enum bool{...
  #endif
  </code></pre>
  
  <p>however this depends on a user that has bool defined also defining _BOOL</p>
  
  <p>Is there a way of checking if types`with specific names already exist?</p>
  
  <p>(Note this is a C Question, not C++, and neither I nor my assumed lib user is using stdbool, Ta)</p>