libpqxx 7.7.1: Choose C++ version yourself. Plus various fixes.

The main change in libpqxx 7.7.1 is that the configure script no longer tries to tell your compiler what C++ language version it should compile. If you want a specific C++ version, you need to pass the right options to the compiler. Otherwise, you get your compiler's default C++ version.

There are many other changes:

• Finally fix a long-standing silly warning in autogen.
• Fix digit_to_number not being found on some compilers. (#518, #519)
• In audit mode, define _FORTIFY_SOURCE to enable some extra checks.
• Make more functions constexpr. Nothing particularly useful though.
• Make more functions noexcept.
• Move constructor & move assignment for result.
• Support LGTM.com and Facebook "infer" static analysis.
• Deprecate set_variable/get_variable on transaction_base. (Design had unearthed warts in SQL variables, which were later fixed.)
• Set/get session variables on connection: set_session_var/get_var.
• Set/get local variables: execute SQL statements.
• When using select(), include <winsock2.h> if available. (#548)
• Library includes its own headers differently. Simpler, compiles faster.
• Fix those pointless MSVC warnings at last.
• Extract waiting functions into separate module.
• Fix when cross-compiling using MinGW and CMake. (#548, #550)

I also reorganised the way libpqxx #includes its own headers. You may find that builds are a bit faster.