title: error: `erfc` is not a member of `std`
body: <p>I am trying to build <em>OpenNN</em> library according to instructions <a href="https://github.com/orian/opennn" rel="nofollow noreferrer">here</a>. But after</p>
  
  <pre><code>make opennn
  </code></pre>
  
  <p>I get the following output (not full):</p>
  
  <pre><code>[2%] Built target tinyxml2 
  [4%] Building CXX object opennn/CMakeFiles/opennn.dir/variables.cpp.o
  In file included from /home/wolfgang/Downloads/OpenNN/opennn/variables.h:32:0,
               from /home/wolfgang/Downloads/OpenNN/opennn/variables.cpp:16:
  /home/wolfgang/Downloads/OpenNN/opennn/vector.h: In member function ‘bool OpenNN::Vector&lt;T&gt;::Lillieforts_normality_test() const’:
  /home/wolfgang/Downloads/OpenNN/opennn/vector.h:1144:20: error: ‘erfc’ is not a member of ‘std’
       Fx = 0.5 * std::erfc((mean - (*this)[i])/(standard_deviation*std::sqrt(2)));
  </code></pre>
  
  <p>the above output ends with</p>
  
  <pre><code>make[3]: *** [opennn/CMakeFiles/opennn.dir/variables.cpp.o] Error 1
  make[2]: *** [opennn/CMakeFiles/opennn.dir/all] Error 2
  make[1]: *** [opennn/CMakeFiles/opennn.dir/rule] Error 2
  make: *** [opennn] Error 2
  </code></pre>
  
  <p>I think I should use <code>-std=c++11</code> or something but I do not know exactly how. I tried to add this line</p>
  
  <pre><code>CXXFLAGS += -std=c++11
  </code></pre>
  
  <p>to <code>Makefile</code> but it does not work.</p>
  
  <hr>
  
  <p>I do not know about <code>cmake</code> and <code>make</code> anything. But thing that those errors were caused by incompatibility with older <code>c++xx</code>.</p>