Problem at hand: dependents (like FairRoot) need to know the
minimum C++ standard level that FairMQ (and its headers)
requires.
The first idea is to let the targets export their
CXX_STANDARD value. First, this doesn't seem to work as
expected.
Second, target_compile_features() seems to be the better
way to go. It has a much better granularity, automatically
has the export feature, and thus should make dependents
behave correctly.
Also drop all of this enforeced CMAKE_CXX_STANDARD*
setting. If it's given, check it. But that's it.
See: https://gitlab.kitware.com/cmake/cmake/-/issues/18446
See: https://cmake.org/cmake/help/latest/manual/cmake-compile-features.7.html#requiring-language-standards
