Enable C++14 for the 2.4 release branch if it is available

There are already two modules that want C++14 support for 2.4 if possible, dune-functions and dune-pdelab. Right now, these modules ship local overrides, but the cleaner method would be to integrate the changes into dune-common and provide the user with an option to disable the C++14 test. See https://gitlab.dune-project.org/pdelab/dune-pdelab/blob/releases/2.4/cmake/modules/PdelabCheckCXX14.cmake for an example