Disable quickcheck-classes when building with stack

Stack doesn't support automatic flags: it doesn't backtrack when
dependency resolution fails using the default value of an automatic
flag, it just fails the build plan construction. We can't use automatic
flags to check if quickcheck-classes is available, and since the code is
tested by the haskell-ci.yml cabal workflow anyway, let's just disable
it here. It's not worth the hassle trying to enable it for select LTS
versions only. It's too much noise already, actually. :-(

Further reading:
https://cabal.readthedocs.io/en/latest/cabal-package.html#resolution-of-conditions-and-flags
https://github.com/commercialhaskell/stack/issues/1313#issuecomment-157259270

stack.yaml

@@ -6,6 +6,13 @@ packages:
 extra-deps:
 - X11-1.9.2
 
+flags:
+  xmonad:
+    # stack doesn't support automatic flags
+    # https://cabal.readthedocs.io/en/latest/cabal-package.html#resolution-of-conditions-and-flags
+    # https://github.com/commercialhaskell/stack/issues/1313#issuecomment-157259270
+    quickcheck-classes: false
+
 nix:
   packages:
     - zlib