Do not exclude `dune` subdirectory from default target

Ansgar Burchardtrequested to merge
ansgar/dune-foamgrid:master into master

While installing dune-foamgrid, we had a strange problem that a different module depending on dune-foamgrid could not find dune-foamgrid. We traced this problem down to make install not installing any header files.

These changes made make install work for us again.

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