We only support up to version 7.0, so don't recommend
upgrading past it. The rest of the documentation around this
was already updated, but one spot was missed.
Document how to build 64 bit Windows binaries using the MinGW64 tool set.
Remove recommendation against using Mingw as a build platform.
Be more specific about when Cygwin is useful and when it's not, in
particular note its usefulness for running psql, and
add a note about building on Cygwin in non-C locales.
Per recent discussions.