[P] add config

Something like in pyMOR, e.g., you can do

from dune.xt.common import config

if config.HAVE_K3D:
    ...

This regards the runtime env. We should also include a selection of the HAVE_ defins from the config.h from compile-time (which can not be influenced any more at runtime). Either

• in the same config as is
• in the same config as BUILT_WITH_...
• in a separate config as is