[Dune] Improve module handling by CMake

[Oliver Sander]

Hi Christoph,

thanks for the detailed explanation.  Is this sequence of steps
something that people agree upon?

> 1. Use CMakeCache.txt from dependencies to avoid re-running checks in
> later modules.

As a cmake non-expert I am surprised by this.  Are the CMakeCache.txt files
really intended to be used like this from third-party software?

You yourself pointed me to those config-file packages, and I wonder why
we cannot use those?  AFAIU these would solve a separate problem:  I have
gotten reports from people that want to use Dune modules like just another
library.  This fails, because Dune modules don't export the relevant information.

> We need to add some extra sorting, as there is not always
> a unique ordering. Take dune-common, dune-geometry and dune-istl. When
> dune-geometry is run prior to -istl, dune-istl must use -geometry's
> cache and not -common's.

Hmm, wouldn't I just merge the two CMakeCache.txt files and discard
duplicate information?

Best,
Oliver

 Dune-grid has to pick up -istl and so on.
> Easiest solution would be alphabetical sorting of names to get a unique
> ordering.
> After this step, dune modules would know the CMake variables in later
> modules.
> 
> 2. Create individual config.h files, for dune-foo this could be
> dune-foo-config.h. For every dependency dune-bar dune-foo-config.h would
> include dune-bar-config.h.  We can stick to config.h, which includes
> dune-foo-config.h.
> After this step, dune modules know the C preprocessor macros.
> 
> 3. Improve handling of linking libraries. CMake offers private and
> public interface propagation.
> After this step, linking and passing additional arguments would be more
> CMake-ish.
> 
> 4. Fix everything that I currently forgot. Get rid of many Dune-ism,
> that one wouldn't write today, that I am even not aware of.
> 
> If others have different ideas, please share them!
> 
> Bye
> Christoph
> 
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