[Opm] A note on upgrading Dune (and possibly other third-party packages)

[Atgeirr Rasmussen]

Dear OPM community,

Yesterday a new version of Dune was released (2.4.1) and I had some problems compiling OPM after updating.
However, the fix is simple: delete your build directories (for opm modules depending on Dune modules, which are
almost all opm modules except opm-common and opm-parser) and reconfigure them with cmake. After that,
everything should be working with the new Dune version.

The underlying issue is that the config.h files that are generated by cmake in your build directories must be updated,
so that the version constants in those files becomes correct.

Atgeirr