title: GCC 4.9.2 on ARMHF missing std::mutex
body: <p>I'm working on an embedded project which will be using an arm7-a (armhf) system on a chip running Debian Jessie.</p>
  
  <p>This system includes GCC/G++ 4.9.2, which <em>should</em> be fully C++11 compatible.</p>
  
  <p>However, I'm seeing issues like, <code>std::mutex</code> and <code>std::condition_variable</code> not being present in the <code>std</code> namespace. I believe this is because the <code>_GLIBCXX_HAS_GTHREADS</code> macro isn't defined. I've found this to be the case whether I pass <code>-std=gnu++0x</code> or <code>-std=c++11</code>.</p>
  
  <p>Does anybody know if this is expected behaviour on this platform?</p>