ghc installation on Linux doesn't like CDPATH

[George Russell]

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.