whoops, fix bug with the refactoring into Types.hs

Derived instances of Read expect all data types to be lexeme readers - that is swallow leading whitespace.
Support for OS specific parts of the build

New OS.hs and data types and implementations (OS/Mac.hs etc...)
Delegate final target paths and some processing to OS type.
  - many changes to GhcDist and Target code

improvements to error messages when config oracles fail to parse
new "virtual dir" style targets (see Dirs.hs)
new </+> operator: place an abs. path under some base