message: toolkit: Link sst_conn_tool directly to core libraries
  The `sst_conn_tool` uses the core APIs directly and does not need to
  link to the public bindings libraries.  Link to the core libraries
  directly and drop use of the `adios2` legacy target.
diff: # accompanying file Copyright.txt for details.
  #------------------------------------------------------------------------------#
  +if(ADIOS2_HAVE_MPI)
  + set(maybe_adios2_core_mpi adios2_core_mpi)
  +else()
  + set(maybe_adios2_core_mpi)
  +endif()
  +
  # stt_conn_tool
  add_executable(sst_conn_tool sst_conn_tool.c sst_conn_tool.cxx)
  -target_link_libraries(sst_conn_tool adios2)
  +target_link_libraries(sst_conn_tool ${maybe_adios2_core_mpi} adios2_core)
  if(WIN32)
  target_link_libraries(sst_conn_tool getopt)
  