This reverts commit 79759fea, reversing
changes made to dc36f4df.

- Update CMakeLists.txt (original template and generated code)
  to prevent forcefully linking `dl` on all UNIX platforms,
  and rely on `${CMAKE_DL_LIBS}` portable variable instead.
- Do not try linking against `librt` on macOS.
- Add a `config_freebsd` directory containing the header file
  generated by the `configure` script for the `ares` library.

add config setting for building c-ares on osx