how do I configure postgresql to find the pg_controldata command
Your shell PATH environment variable is not pointing to your folder /usr/lib/postgresql/9.4/bin/.

In order to do this, use the command PATH="/usr/lib/postgresql/9.4/bin/:$PATH".

If you want to automate this, you can add this line in your shell config file (ie. ~/.bashrc) with this :
echo 'PATH="/usr/lib/postgresql/9.4/bin/:$PATH"' &gt; ~/.bashrc.