Allow to skip dune_python_package_install when pip is not found.

dune_python_package_install can be skipped if pip is not found if the PYTHONPATH is set manually. This MR adds a flag that allows to do that.

Hmm, I do not understand what exactly you need this for. What exactly is it you provide through your manually set PYTHONPATH?

Also, I doubt that the given implementation does what you claim, as you unconditionally override the value of DUNE_PYTHON_INSTALL_PACKAGE_REQUIRE_PIP set by the user (I havn't tried it, but that is how I read the code).

@dominic: The value DUNE_PYTHON_INSTALL_PACKAGE_REQUIRE_PIP should be TRUE unless specified via -DDUNE_PYTHON_INSTALL_PACKAGE_REQUIRE_PIP=FALSE. I have tested it and it works. Do you have a better solution for this?

In general this flag allows to skip the check for pip which I think is essential since things work without it. The default behaviour is unchanged.

assigned to @andreas.dedner

mentioned in merge request !399 (merged)

@robert could you check !399 (merged) it's quite a simple approach which might be better for 2.6 - but I couldn't test since I have pip...

closed