That is the way it is done in other modules, too. We need this
at least for libecl as this target is used in opm-common_LIBRARIES
and not defined if there was no find_package(ecl) call. That call
is now made during execution of find_package_deps(opm-common).
use pure config mode.
we no longer compile a test application for the libraries,
however checks have been tightened by making sure version of
all opm modules are the same.
this allows for putting prereqs directly in the module repositories
