Fixes two platform-specific build problems in 7.8.0:
Microsoft Visual Studio had been complaining loudly in C++20 mode when we included the deprecated header <ciso646>. But without that header, it turns out that Microsoft Visual Studio in C++17 mode can't compile libpqxx. So now we include the header only when compiling in C++17 mode.
The configure script built using autoconf did not support Apple's M1 and M2 ARM CPUs. Regenerating the script with a newer autoconf fixed that.