title: Can't pip install GDAL for python 3.6 on mac os
body: <p>I can pip install GDAL for the python2.7 on my mac just fine using pip. However when I try to install the package for my python3.6 using 
  <code>pip3 install gdal</code> I get a long error message that ends with this:</p>
  
  <pre><code>warning: unknown warning option '-Werror=unused-command-line-argument-hard-error-in-future'; did you mean '-Werror=unused-command-line-argument'? [-Wunknown-warning-option]
      extensions/gdal_wrap.cpp:3168:10: fatal error: 'cpl_port.h' file not found
      #include "cpl_port.h"
               ^~~~~~~~~~~~
      1 warning and 1 error generated.
      error: command 'clang' failed with exit status 1
  </code></pre>
  
  <p>I tried re-installing GDAL using homebrew as suggested here, <a href="https://stackoverflow.com/questions/24102437/unable-to-install-gdal-in-python-3-using-pip-clang-failed-with-exit-status-1">Unable to install GDAL in python 3 using pip (clang failed with exit status 1)</a>, but no luck.  Looking for any suggestions. </p>