Fallback to standard cabal location on OS X

If ~/Library/Haskell does not exist, fall back to ~/.cabal/bin.

Ideally, we'd parse ~/.cabal/config here, but cabal does not provide an
interface to get configuration settings.

modules/haskell/init.zsh

@@ -11,11 +11,10 @@ if (( ! $+commands[ghc] )); then
 fi
 
 # Prepend Cabal per user directories to PATH/MANPATH.
-if [[ "$OSTYPE" == darwin* ]]; then
+if [[ "$OSTYPE" == darwin* && -d $HOME/Library/Haskell ]]; then
   path=($HOME/Library/Haskell/bin(/N) $path)
   manpath=($HOME/Library/Haskell/man(/N) $manpath)
 else
   path=($HOME/.cabal/bin(/N) $path)
   manpath=($HOME/.cabal/man(/N) $manpath)
 fi
-