title: Using boost::filesystem fails due to "_fchmodat" on mac
body: <p>I want to use some functions of <code>boost::filesystem</code>.</p>
  
  <p>I can include <code>boost/filesystem.hpp</code>and <code>boost/filesystem/path.hpp</code> without any problems, but as soon as I use some specific functions as <code>boost::filesystem::current_path()</code> or <code>boost::filesystem::initial_path()</code> I get the following compiler error:</p>
  
  <pre><code>Undefined symbols for architecture x86_64:
  "_fchmodat", referenced from:
  boost::filesystem::detail::permissions(boost::filesystem::path const&amp;,
  boost::filesystem::perms, boost::system::error_code*) in 
  libboost_filesystem.a(operations.o)
  ld: symbol(s) not found for architecture x86_64
  </code></pre>
  
  <p>I am linking against <code>-lboost-system</code> and <code>-lboost-filesystem</code> and I'm using <code>-std=c++0x</code> as compiler tag.</p>
  
  <p>The version of <strong>boost</strong> is 1.56.0 installed via homebrew.</p>
  
  <p>Edit: I also tried it using <strong>boost 1.57.0</strong> but the error is the same.</p>