Specify BOOST_SRC_VERSION to locate source download, extract BOOST_VERSION from source

Commit message: ["Instead of specifying BOOST_VERSION which applies to both the download URL and the source code itself, now specify BOOST_SRC_VERSION. BOOST_SRC_VERSION specifies where to download the source (e.g. the source tarball download link), and once it is downloaded and extracted, the boost.sh script extracts the actual library version from <src-dir>/boost/version.hpp. The only effect this change has is to request the source version via a different environment variable name. The purpose of this feature is to support the subsequent addition of a feature to optionally retrieve the Boost source code from the Boost Subversion repo by specifying either a repo revision number or the "HEAD" special value. These specifiers have no information about the library version, so in these cases the actual library version *must* be extracted from the Boost library source code."]