windows build docs

Following recent discussions and the enabling of 64 bit Mingw builds, I 
propose to make the attached changes to the docs. I don't see any great 
reason for us to advise against building with Mingw, especially now that 
we have 64 bit support for it, so I removed that, amd also clarified 
where Cygwin is useful and where it's not, as well as adding some detail 
about how to make 64 bit builds.

cheers

andrew