Quadmath support breaks compilation with clang when using dune_enable_all_packages()

The CMake test for quadmath adds a library to the Dune package database by calling dune_register_package_flags(), but does not provide a corresponding add_quadmath_flags() or something similar. For that reason, nothing in dune-common tests whether these flags actually work.

I just stumbled over this trying to rebuild the CI images for PDELab (which uses dune_enable_all_packages() and thus pulls in the quadmath library definition). Unfortunately, in addition to adding the required linker flags, the CMake test also adds -fext-numeric-literals to the compile options, which is a GCC extension not understood by clang, causing the build to fail, cf. https://gitlab.dune-project.org/docker/ci/-/jobs/44285.

I think we need to do two things:

• Add a test to dune-common that actually uses the quadmath library as defined in dune_register_package_flags().
• Check that the compiler actually understands -fext-numeric-literals. I have no idea whether it is better to disable the whole feature if the option is missing, or whether we should just drop the compile option in that case.