title: got compile error when use clock_gettime in c99
body: <p>when I use clock_gettime in my code snippet, and compile with flag -std=c99, I got a error like this:</p>
  
  <pre><code>warning: implicit declaration of function 'clock_gettime'
  error: 'CLOCK_REALTIME' undeclared (first use in this function)
  </code></pre>
  
  <p>and I have included the file 'time.h'.
  anyone knows how to fix it.</p>