libpqxx 6.4.8: bug fix in old release series

This bugfix update to the old 6.x series resolves a bug where executing a prepared statement would fail to activate the ongoing transaction.

That is, if the first thing you did in a transaction was execute a prepared statement, the actual transaction would not start at that point. The prepared statement would execute outside it.

Newer versions of libpqxx (as of 7.0) never had this problem because they start transactions as soon as you create them. If your environment supports C++17, please use those newer versions instead.