For single build-type generators such as unix make files check
CMAKE_CXX_FLAGS_<CONFIG> in addition CMAKE_CXX_FLAGS.  This is equivalent to
just CMAKE_CXX_FLAGS if not build-type is selected.  For multi-configuration
generators, check each configuration, and skip the test is any of them is bad.

I'd like to skip it only for the bad configs, but that would require generator
expression support in dune_add_test(CMAKE_GUARD ...).