DCMTK

The current CMake setup "heuristically" determines compiler flags (e.g. CMAKE_CXX_FLAGS) based on several factors and potentially overwrites any user settings. Newer versions of CMake provide a better mechanism to do so and already ship with heuristics for various compilers, potentially better than the ones we provide with DCMTK: https://cmake.org/cmake/help/latest/manual/cmake-compile-features.7.html . DCMTK's CMake setup should be modified to use cmake_compile_features() et al., while ensuring that existing convenience functionality provided by the custom scripts keeps working.