Try to use stack instead of cabal.

See #110. [skip ci]

.appveyor.yml

 clone_folder: "C:\\msys64\\home\\ghc\\shake-build"
 
-# cache:
-#    - "C:\\msys64\\home\\cabal"
-
 install:
     - set MSYSTEM=MINGW64
     - set PATH=C:\msys64\mingw64\bin;C:\msys64\usr\local\bin;C:\msys64\usr\bin;%PATH%
-    - bash -lc "pacman -S --noconfirm git binutils p7zip gcc"
+    - curl -ostack.zip -L --insecure http://www.stackage.org/stack/windows-i386
+    - 7z x stack.zip stack.exe
+    - stack exec -- pacman -S --noconfirm gcc binutils p7zip git
+    - stack install -v3 -j --prefix=/usr/local alex happy
     - bash -lc "curl -LsS http://www.haskell.org/ghc/dist/7.10.1/ghc-7.10.1-x86_64-unknown-mingw32.tar.xz | tar -xJ -C /mingw64 --strip-components=1"
-    - bash -lc "mkdir -p /usr/local"
-    - bash -lc "mkdir -p /usr/local/bin"
-    # - bash -lc "curl -LsS https://www.haskell.org/cabal/release/cabal-install-1.22.0.0/cabal-1.22.0.0-i386-unknown-mingw32.tar.gz | tar -xz -C /usr/local/bin"
-    # - bash -lc "mv /usr/local/bin/cabal-1.22.0.0-i386-unknown-mingw32.exe /usr/local/bin/cabal.exe"
-    - bash -lc "curl -LsS https://www.haskell.org/cabal/release/cabal-install-1.20.0.3/cabal-1.20.0.3-i386-unknown-mingw32.tar.gz | tar -xz -C /usr/local/bin"
-    - bash -lc "cabal update -v3"
-    - bash -lc "cabal install -j --prefix=/usr/local alex happy"
     - git clone --recursive git://git.haskell.org/ghc.git C:\msys64\mingw64\home\ghc\tmp
     - bash -lc "mv /home/ghc/tmp/* /home/ghc"
     - cd C:\msys64\mingw64\home\ghc\shake-build
-    - bash -lc "cd /home/ghc/shake-build && cabal install --only-dependencies"
+    - stack install --only-dependencies
 
 build_script:
     - bash -lc "cd /home/ghc && ./boot"