Fix #289, #279; Windows-specific changes for 8.2.2, 8.4.2

[randen]

Fix issue #289, Add command line switches to hptool for Windows Fix issue #279: add MSys to user's cabal.config

NOTE: The changes for adding/changing the command line switches will affect the other build environments. Issue #289 includes the changes, but in summary, the --Xc, --Xf, --Xg, --Xs, --Xx will likely be needed, replacing the positional (and some flags) params from before.

NOTE: There are some changes here which aim to help allow shake to run parallel actions. Using shake's "-j" switch (which hptool accepts, as it is shake which processes the arguments first) can control how many parallel threads are used. Also, cabal will run its own build steps in parallel when it can, depending on its own "jobs" parameter from the user's cabal config file.

[randen]

(Mentioning, with Github-recognizable syntax, the haskell/cabal#5362 issue.)