Old boost versions lack of generic TLS support

The 2017/03/05 a "generic TLS" method was added to boost/asio/ssl/context_base.hpp which maps the OpenSSL TLS_method() described in the SSL_CTX_NEW manual page.

This ticket does two things:

as an emergency measure switch to tlsv12 from tls.
detects the support of the boost::asio::ssl::context::tls method in configure

Note the documentation should be updated to recommend to use a boost version with the support i.e. at least 1.64.0

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