mingw64

With the attached patch I have managed to get Postgres built and running 
a clean set of regression tests using the Mingw64 toolset on my 64bit 
Windows7Pro machine.

There's lots of work still to do (lots of warnings to make go away, for 
example), but this is a pretty encouraging start.

It should also answer most of the questions XiaoboGu had about how to 
build the client libraries, since this falls out along the way.

NB: on Windows 7 you really need to build and run as a non-admin user. 
Windows blows away the PATH for your app otherwise. This took me a while 
to get to the bottom of.

cheers

andrew