title: xmlStrEqual when import igraph.Graph - linking libxml2
body: <p>I am trying to import <code>igraph.Graph</code> from the python-igraph(0.7.1.post6) package using python 3.5.0 on Ubuntu 16.04.1 LT but get an error:</p>
  
  <pre><code>from igraph._igraph import *
  
  ImportError: /home/ubuntu/djangoproject/env/lib/python3.5/site-packages/igraph/_igraph.cpython-35m-x86_64-linux-gnu.so: undefined symbol: xmlStrEqual
  </code></pre>
  
  <p>My understanding is that the package requires <code>xmlStrEqual</code> from libxml2, but that no binding has been made. Libxml2 is installed on my system. Running <code>$apt-get install libxml2</code> gives:</p>
  
  <pre><code>libxml2 is already the newest version (2.9.3+dfsg1-1ubuntu0.1).
  </code></pre>
  
  <p>I am using a virtualenv with the following setup like so:</p>
  
  <pre><code>virtualenv -p python3 env
  source env/bin/activate
  pip install python-igraph==0.7.1.post6
  </code></pre>
  
  <p>Running <code>$ ldd /home/ubuntu/djangoproject/env/lib/python3.5/site-packages/igraph/_igraph*.so</code> gives:</p>
  
  <pre><code>linux-vdso.so.1 =&gt;  (0x00007ffdd33b8000)
  libm.so.6 =&gt; /lib/x86_64-linux-gnu/libm.so.6 (0x00007f7ce24c5000)
  libstdc++.so.6 =&gt; /usr/lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007f7ce2143000)
  libgcc_s.so.1 =&gt; /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007f7ce1f2c000)
  libpthread.so.0 =&gt; /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f7ce1d0f000)
  libc.so.6 =&gt; /lib/x86_64-linux-gnu/libc.so.6 (0x00007f7ce1946000)
  /lib64/ld-linux-x86-64.so.2 (0x000055e56299f000)
  </code></pre>
  
  <p>I would expect a binding such as <code>libxml2.so.2 =&gt; /usr/lib/x86_64-linux-gnu/libxml2.so.2</code> also.</p>
  
  <p>Does anyone know why this isn't happening or a solution to this problem?</p>