I decided to look around. 11.0 got released 2 years ago. I decided to go with 11.x postgresql with 7.0 libpqxx. The stable branch came out around same time as 11.0 so I will go with probably 11.3 and 7.0.
I installed 12.0 development branch on postgresql with mingw. It is too new to use with libpqxx. Anyone have a recent version of postgresql working with libpqxx? What is a good version?