symlink-bindir does not play nice with program-suffix

(Imported from [Trac #924](http://hackage.haskell.org/trac/hackage/ticket/924), reported by guest on 2012-03-05)

I'd expect symlinks to be created with the program-suffix. Instead what happens is that cabal-install correctly suffixes the installed binary, but attempts to create a symlink to it from an unsuffixed name, which can fail if the unsuffixed name already exists in the symlink-bindir.

~ $ cabal --version

cabal-install version 0.13.3

using version 1.14.0 of the Cabal library

Example:

gruff $ cabal install --program-suffix=-fixed

...[snip]...

Warning: could not create a symlink in /home/claude/opt/bin for gruff because

the file exists there already but is not managed by cabal. You can create a

symlink for this executable manually if you wish. The executable file has been

installed at /home/claude/.cabal/bin/gruff-fixed

(The symlink already existed in this particular case, because I wanted to install my development program with a suffix to avoid clobbering the known working version.)