Better GHC builds for Windows users

[Austin Seipp]

Hello all,

After fiddling around this weekend I've written a quick, comprehensive
guide to a Better Build Environment for windows users. It pretty much
"Just works" and I think should make it easy to get started with new
machines:

https://ghc.haskell.org/trac/ghc/wiki/Building/Preparation/Windows/MSYS2

Many thanks to Kyra for pointing this out to us on the GHC developer
list. It is a much nicer to use mingw/msys environment with a lot of
bugs fixed, and it's easy to install. Following the instructions
should mostly be copy-paste.

In particular, this resolves some weird issues with `validate` and
traditional `make`, including a horrible deadlocking bug with make
(and also 'tee', it seems.) So windows users can build in parallel
again and everything works as expected.

It also gives a simple and complete guide to build for x86_64 users
(we didn't really mention it previously.)

Please shout if you have problems. I've tested this with both an x86
and x86_64 build.

-- 
Regards,

Austin Seipp, Haskell Consultant
Well-Typed LLP, http://www.well-typed.com/