- Turn off USE_DISCORD_RICH_PRESENCE because the binaries are not always
compatible with Linux build.
- Update build instructions for Linux, Windows and macOS
- Update ext-boost for fixing missing headers
Currently the ext-boost repo contains:
- Boost.ProgramOptions
- Boost.Filesystem
- Boost.System
No direct usage for the last one yet, but it may be and it was already
a dependency by another lib anyway, so I included it in the "guaranteed"
libraries in the `bcp` command.