[Dune] first release candidate for 2.6 release

[Christian Engwer]

> OTOH, CMake 2.8 in particular has a whole bunch of weird little bugs and subtle
> differences from CMake >= 3.1 (not accepting keyword arguments in some places where
> later releases will flag a deprecation warning if you leave them away for example). And configuring
> different modules at different compatibility levels is just an invitation for horrible small problems, mostly
> because our downstream modules all re-run the CMake code of upstream modules.

Sorry, but this means the whole buildsystem is broken. The implication
of what you just said is, that we have to use the most recent cmake,
as some downstream module might use it.

I'm happy bumping hte requirements, if there is a particular reason,
but not just some vague "little bugs and subtle differences". That
cmake is strange is a problem for a long time and it will be like this
also for an other long time. I there is a particular bug we fixed and
thus had to raise the requirements, then the discussion is settled and
we have to live with cmake-3.1. It is just, that nobody up to now
mentioned a particular reason for the new requirements and I couldn't
find any hint in the logs.

Christian