title: add default path of GSL for gcc(g++) compiler on Linux
body: <p>system: Ubuntu 14.04
  I download a GSL(GNU Scientific Library) and it is correctly installed on my computer, in directory /home/chenjiyao/GSL </p>
  
  <p>Then I add a few lines at the buttom of <code>~/.bashrc</code></p>
  
  <p><code>export PATH=$PATH:/home/chenjiyao/GSL/bin</code></p>
  
  <p><code>export C_INCLUDE_PATH=$C_INCLUDE_PATH:/home/chenjiyao/GSL/include</code></p>
  
  <p><code>export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/home/chenjiyao/GSL/lib</code></p>
  
  <p>then I just <code>source ~/.bashrc</code> in the terminal.</p>
  
  <p>But it seems not working because when I <code>include&lt;gsl/gsl_math.h&gt;</code> in the source file, I was told that the head file is not found, which means the path for gcc is not settled correctly.</p>
  
  <p>What's wrong with it ?</p>
  
  <p>Thank you.</p>