The library support in dune_enable_all_packages (more specifically, the
ability to add source files to an existing library target with
dune_library_add_sources) relies on target_sources, which only exists in
CMake 3.1+.

To avoid confusing users, this patch emits a meaningful error for older
versions of CMake and generates a warning for more recent versions. This
warning can be disabled by defining
DUNE_ENABLE_ALL_PACKAGES_SILENCE_LIBRARY_WARNING.