Reduce dependence on `PQXX_HAVE_CXA_DEMANGLE`.
Reduce dependence on `PQXX_HAVE_CXA_DEMANGLE`.

I've been trying to SFINAE my way around the need for this macro
altogether, so that I could do without the configure check.  But,
no louck so far.

jtv/libpqxx,jtv/libpqxx,jtv/libpqxx,jtv/libpqxx