- Add __config_site for NuttX.
In libcxx 12.0.0. CMake concatenated __config_site with _config,
which enabled NuttX to build without its inclusion. In 15.0.7, this
file is configured by CMake and included explicitly in __config.
- Add additional include directories.
- Mute always_inline warning.
- Make the download of libcxx/libcxxabi configurable