title: Can't compile C++11 source using GCC 5.1 toolchain
body: <p>I'm trying to compile a library written with C++11 features using GCC 5.1 on Ubuntu. However, it complains <code>std::unique_ptr</code> is not defined. </p>
  
  <pre><code>gcc (Ubuntu 5.1.0-0ubuntu11~14.04.1) 5.1.0
  g++ (Ubuntu 5.1.0-0ubuntu11~14.04.1) 5.1.0
  </code></pre>
  
  <p>CXX flags:</p>
  
  <pre><code>-std=c++11 -Wall -Wextra -Weffc++ -pedantic
  </code></pre>
  
  <p>Output:</p>
  
  <pre><code>error: ‘unique_ptr’ in namespace ‘std’ does not name a template type
           std::unique_ptr&lt; detail::RegexImpl &gt; m_pimpl;
  </code></pre>
  
  <p>However, I'm able to compile the exact same code on OSX.</p>
  
  <pre><code>Apple LLVM version 6.1.0 (clang-602.0.49) (based on LLVM 3.6.0svn)
  </code></pre>
  
  <p>CXX flags:</p>
  
  <pre><code>-stdlib=libc++ -std=c++11 -Wall -Wextra -Weffc++ -pedantic
  </code></pre>
  
  <p>What am I doing wrong? </p>