title: Python GDAL package missing header file when installing via pip
body: <p>I'm trying to install gdal from pip <code>pip install gdal</code> inside a virtual environment (Ubuntu). It fails because it cannot find <code>cpl_port.h</code></p>
  
  <pre><code>extensions/gdal_wrap.cpp:2853:22: fatal error: cpl_port.h: No such file or directory
  compilation terminated
  </code></pre>
  
  <p>However GDAL is installed correctly and the header file is located at <code>/usr/include/gdal/cpl_port.h</code>. Is there some environment variable for GDAL that needs to be set in order for pip to find the header files?</p>