That's needed in files used to build the configure program.
For the other files, I'm keeping my __WINDOWS__ define, as I find that much easier to type.
I may search and replace it to use the mingw32_HOST_OS thing later.
The only thing lost is ./ghci
Speed: make fast used to take 20 seconds here, when rebuilding from
touching Command/Unused.hs. With cabal, it's 29 seconds.
