ghc installation on Linux doesn't like CDPATH

The "cd" command on Linux considerately prints out the name of the
new directory, which it gets from the CDPATH. This has the unfortunate
side-effect of breaking "gmake install" for ghc6.0, which uses some shell
hackery of the form something like:

for i in `(cd share ; find .)`

where i's first value will of course be the new directory printed out by
"cd".

The workaround is to set CDPATH to null.