Update demo and manual to use Stack GHC

[bhipple]

Since the docs recommend installing Shake with stack, a raw command to
ghc will likely fail due to missing libraries if a user tries to re-run
the Shake demo after the initial stack exec -- shake --demo cmd.

[ndmitchell]

I tend to use if type stack > /dev/null 2>&1; then as the guard here. Any reason to prefer your formulation or mine? (I'm not a big shell scripter, so defer to your knowledge here!)

[ndmitchell]

Thanks! Although I slightly wonder if the guidance should be run to stack exec -- build and keep the stack bit outside of Shake? I would rather Shake didn't have to pick a side in the Cabal vs Stack thing, but maybe it's just something I can't avoid?

[bhipple]

That's a good point; keeping the build.sh script as simple as possible seems preferable. In my first commit I just had used Stack, since that's what the documentation suggests. Once I realized Travis was using Cabal it got uglier.

Regarding type, it'll most likely work everywhere, but Shellcheck points out that it's not POSIX standard. It is much cleaner though.

[ndmitchell]

Ah, interesting about ShellCheck - I haven't run it on that particular script for a while.

Looks good, but on Windows it doesn't work - commercialhaskell/stack#2225 - I think this is a Stack bug so we'll see what they say before going forward. If it looks like it will be difficult to solve on their side we can have different Windows/Linux instructions, but I'd rather avoid that if we can.

[ndmitchell]

Stack have now fixed so stack exec build works on Windows. All merged and switched to build.bat on Windows (mostly for better symmetry with build.sh). Since the code to run build.bat is only in the dev version of Stack I'll hold of regenerating the website right now and leave it a little bit. Thanks for the patch!