[boost] Embarcadero PRs

I have PRs for these libraries, which have passed CI testing, for distinguishing the Embarcadero C++ clang-based compilers from the legacy bcc32 compiler: beast thread regex container python program_options units circular_buffer graph parameter unordered random typeof Can the maintainers of these libraries please merge the appropriate PR ? The PR should not affect any other compiler, and it gives a chance for the Embarcadero C++ clang-based compiler(s) to use the appropriate Boost library even when some lacking feature or Embarcadero bug currently prevents the full functionality of the library to be used.