For the "extract version from source" feature, revert the public-facing variable name from the changed BOOST_SRC_VERSION back to the original BOOST_VERSION. The attempt to clarify its modified purpose was not very successful, certainly not to the extent that it justified changing the public interface to the script. The internal variable used by the script is now BOOST_HPP_VERSION.
Merge branch 'extract-version-from-source' into source-from-boost-repo