title: PostgreSQL error when trying to create an extension
body: <p><code>ERROR:  could not open extension control file "/usr/share/postgresql/9.1/extension/postgis.control": No such file or directory</code></p>
  
  <p>Is what I get when I try to <code>CREATE EXTENSION postgis;</code> can anybody tell me why is it so and what can I do to fix it?</p>