I was not sure if this qualifies as a bug, so I thought I would send it in here first.
The EPEL repo for EL7 contains a package libpqxx-devel-4.0.1-1, which (in my view incorrectly) puts a dependancy on postgresql96-devel, meaning you can only install it with postgresql 9.6.
The EPEL repo for EL6 contains libpqxx-devel-4.0.1-2 which has this issue fixed - it instead depends on postgresql-devel (which is provided by any postgresql level). I believe this is the correct relationship.
Does anyone know why the -2 version was not built / deployed for EL7?