Update windows build documentation

[tobil4sk]

Opam runs natively on Windows now and manages cygwin automatically, so there is no need to install cygwin manually anymore. These instructions explain how to set up opam like this.

A workaround is currently needed for luv, see: aantron/luv#162

[Simn]

I still haven't tried this locally and I'm scared to do so, but if you say that this is the new way then I'll believe you. Let me know when this is good to merge.

[tobil4sk]

I still haven't tried this locally and I'm scared to do so

I think if you check out a new copy of the haxe repository for testing and get the native opam to manage its own separate cygwin install, it should be possible to test this in a way that you can just delete the %LOCALAPPDATA%\opam folder (and remove the PATH entry) if anything goes wrong and return to your current setup.

Let me know when this is good to merge.

I'm wondering if there is a better way to expose make to the build environment other than adding the cygwin bin directory to PATH, but maybe that's for the future.

Perhaps we should wait until someone else follows the instructions to double check I didn't miss anything.

[ncannasse]

This fails with:

/usr/bin/tar: Child returned status 127
/usr/bin/tar: Error is not recoverable: exiting now

[tobil4sk]

What is the output if you run these commands:

opam exec -- cygpath -w "/bin/tar"
& (opam exec -- cygpath -w "/bin/bash") -c "type tar"