Use short paths for extra-prog-path in cabal's config

[randen]

One of the last steps in the HP installer is to invoke the just-installed cabal to modify its config file to write the extra-prog-path, extra-lib-dirs., and extra-include-dirs.. values with directories from the installed locations of MSYS binaries/libraries. Some executables from our MSys distro do not like spaces in the PATHs. So, we should use the MS-DOS-style "short names" for the entries we ask cabal to write to its config file (e.g., short path: "C:\Progra~1\Haskel~1\lib" rather than the long path containing spaces: "C:\Program Files\Haskell Platform\lib").