Safe-guard against version mismatch

The version declared for the build system (in dune.module) is checked
against what is in the source (e.g. opm/core/version.h) and if these
don't match, then issue an author warning.

This will help us keep the two version numbers in sync., since it
becomes very obvious when we don't. The benefit for this is to not have
the build system start mucking with the code just to dump some static
information in there.