Windows building instructions

[Simon Marlow]

I've made a start on sanitising the Windows building instructions, and 
generally reoganising the Windows-related stuff in the wiki.  Basicalliy my 
plan is to:

  - Simplify the instructions.  In lots of places we have stuff like "if you
    get the following bizarre error message ... then do this ...".  I'd
    rather have a set of simple instructions that just work, with a separate
    (searchable) page for troubleshooting advice.  So now we have

    http://hackage.haskell.org/trac/ghc/wiki/Building/Troubleshooting

    which is just a list of error messages and solutions, collected from
    various other places in the docs.  If you encounter an error that isn't
    listed on that page, please add it (guidelines for adding stuff are at
    the top).

  - Merge the Windows-specific instructions with the rest of the docs, so
    we can avoid repetition.  So now the instructions for how to set
    up your Windows system for building GHC are on the main "prerequisites"
    page:

    http://hackage.haskell.org/trac/ghc/wiki/Building/Prerequisites

  - I've uploaded a set of build tools that work (just MSYS for now) to
    haskell.org, and linked them from the wiki.  This morning I started
    from a fresh Vista system, installed these tools, and managed to
    do a complete validate of the GHC HEAD, with 1 test failure.


What we still need to do:

  - Expand the instructions to include Cygwin.  I've linked to the
    existing Cygwin instructions for now, but I think they need
    simplifying.

  - Add more common failures and troubleshooting tips.

  - Merge in contrib docs, e.g.

    http://hackage.haskell.org/trac/ghc/wiki/ProblemsCompilingGhc

I'd be grateful for any help with this - just edit away!

Cheers,
	Simon