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.
It gets tiresome to rebuild the project everytime one makes changes to
any of the unit tests. We don't likely publish results based on the
debug code anyway, so tracking the version number there is not such a
pressing issue.
Otherwise, the library may be declared "done" before the version number
process have had any change to touch files it depends upon. This ensures
that the version number is correct before we start building the library.
Akin to config.h, we write project-version.h whenever the VCS sha hash
of the project changes. This file can then be included to embed this
into the project. Since this changes more frequently, we choose another
file than config.h. Care is also taken to not rewrite the header if the
information doesn't change, but the project is rebuilt.
