libpqxx 6.4.7: Patch update to last pre-C++17 release

By request, here's an update to the last 6.x release. It fixes a link error where, when building libpqxx as a shared library, you might not have the nullptr_t specialisation of extract_value in stream_from.

This is in response to #340, but it may help others with compilers which don't support C++17.