[Dune] Doxygen documentation with make install

[Christian Engwer]

> - In Addition, dune-web creates an overall documentation for
>   all the packages it was configured with (yes, I implicitly
>   propose to autotoolize dune-web here) and installs it in
>   a separate directory.

I object!

I don't think we need to autotoolize dune-web. At the current stage
this will lead to bisg problems, as we will egt cyclic
dependencies. Currently these don't metter, as dune-web only needs the
source code.

Christian