[WIP] Be compatible with dune-common 2.6

this is a workaround for the mpi4py dependency issue
where it cannot both be installed as a python subdir dep and
be present or cmake to find header install paths to add

Visibility updates

Adding this macro in your source tree with a list of module names
will give you a 'dependent_bindings' target to build and if you don't
set ```-DDXT_NO_AUTO_BINDINGS_DEPENDS=TRUE``` this is added as a
dependency for the local 'bindings' target

[cmake] use dune's venv interpreter instead of finding our own