Make use of obabel installed next to Avogadro

This is often the default when using an installer/package directory,
ensure that we do not do this if installed in /usr/*.

Change-Id: I83aca24be665abe68815182cbed96b303e90dd44
Merge topic 'openbabel-relative' into master

08da46f1 Make use of obabel installed next to Avogadro