Merged requested to merge fix/cmake-python-enable-shared into master
!3436 (merged) accidentally removed DUMUX_CMAKE_FLAGS which was used in some downstream tests to set BUILD_SHARED_LIBS for Python. The fact that is was removed shows that it wasn't clear what it's good for. We need a better mechanism that actually enforced our requirement of BUILD_SHARED_LIBS=ON when using Python bindings. This attempts to do this through CMake.
We also enforce C++17 by setting it as a PUBLIC interface of the dumux library.