put platform on OS X at /Library/Haskell/ghc-$ghcver-$arch

This is because cabal uses the "ghc-" prefix as part of the $compiler template variable

fix up activate-hs
fix uninstall-hs to support versions of the form 7.8.2.20140628
improve cabal.wrap

update to new default cabal config file as basis of the config cabal.wrap writes
improve logic by which it decides to update the config and config.platform files
include a message if config.platform was updated, but config was not because it was user modified
simplify the ghc-clang-wrapper

remove self installation and settings file fix up logic -- this is now handled by activate-hs
build notes update