title: M_PI not available with gcc --std=c11 but with --std=gnu11?
body: <p>I noticed <code>M_PI</code> is unavailable on <code>c11</code>. By looking at <code>/usr/include/math.h</code> I can see <code>M_PI</code> is defined if: </p>
  
  <pre><code>#if !defined(__STRICT_ANSI__) || ((_XOPEN_SOURCE - 0) &gt;= 500)
  ...
  #define M_PI 3.1415...
  #endif 
  </code></pre>
  
  <p>Moreover in the <code>math.h</code> from <a href="http://sourceware.org/git/?p=glibc.git;a=blob;f=math/math.h;h=c7ff16971e98662081e3192a1269038e9419af59;hb=refs/heads/master" rel="noreferrer">glibc</a> <code>__STRICT_ANSI__</code> is replaced with <code>__USE_MISC</code>. I am completely lost with this. </p>
  
  <p>What is the story in between <code>--std=c11</code> and the constants defined in <code>math.h</code>?</p>
  
  <p>Which <code>libc</code> should I consider on a <code>debian</code> distribution ?</p>
  
  <p>By the way, <code>M_PI</code> is defined in <code>c99</code> and <code>gnu11</code>...</p>