cabal install won't replace symlink to a different path

Cabal refuses to replace symlinks in `symlink-bindir` unless it detects it installed the symlink itself. The heuristics to detect this is too restrictive, and breaks if the new symlink should point to a different path.

This happens when the prefix is not fixed, for instance on the default install of the Haskell Platform for Mac OS X.

_EDIT_: avoid the term "symlink target" since it's ambiguous (the man pages of ln(1) for the BSD and GNU versions use conflicting terminology).