title: Possible to enable libc++ debug checks?
body: <p>I've been wondering if it is possible to enable any debug checks for libc++. One of my favorite things about MSVC's STL is that it catches some otherwise hard to find bugs right from the get go (though I wish it were easier to disable for speed).  After peeking in the headers I tried:</p>
  
  <pre><code>#define _LIBCPP_DEBUG_LEVEL 2
  </code></pre>
  
  <p>However, this leads to build errors ('__get_db undeclared').  Any insights on whether this is a work in progress, or if there is a different expected way to enable these checks?</p>