Open babel doesn't find eigen2 in avogadro-squared build


When building avogadro using avogadro squared, open babel doesn’t find
the eigen2 include files. I pushed a patch to gerrit to include the
new conformer options from OB but these require OB to be build with
eigen2. I think we need to add the eigen2 path to avogadro_build_vars
before including External_openbabel but I’m not sure how this all
works yet.