Set the DUNE_PYTHON_WHEELHOUSE pyth even if CMAKE_INSTALL_PREFIX is set after the first run

summary

The variable DUNE_PYTHON_WHEELHOUSE contains the path where python files from the build-dir are installed to. This is a cache variable initialized to ${CMAKE_INSTALL_PREFIX}/share/dune/wheelhouse. The problem is that if CMAKE_INSTALL_PREFIX is set after the first cmake run, this variable is not updated since cached. The effect is, that wheel files will be installed in a read-only system location, leading to a cmake error, see #339 (closed). The MR solves this issue by creating a meaningless value first and setting it afterwards.

Closes #339 (closed).

added buildsystem python labels

requested review from @andreas.dedner

@simon.praetorius I don't know much (or anything) about the wheelhouse setup so anything you do here is fine with me. dune-testing makes use of them (and probably needs to) so if that passes with this change I would think everything is fine. I'm almost sure that none of the github tests use them.

For context (at least as far as I understand Dominic's concept): the wheelhouse allows you to install dune modules into the system from source and then at a later stage install their Python bindings using the wheels. The nightly build uses this e.g., after testing dune-common the wheel is installed together with the sources. When testing dune-geometry using a new pipeline one has the installed sources for the C++ parts and can use the wheel to install the Python bindings for dune-common. I never have this setup where I install some upstream packages from source but for dune-testing that is needed I believe and there might be other use cases.

resolved all threads

merged

mentioned in commit a36b19af

mentioned in merge request !1259 (closed)

mentioned in issue #358

mentioned in merge request !1417

mentioned in issue #378