Update docs on building for Windows to accomodate current reality.

Update docs on building for Windows to accomodate current reality.

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.