lots of cvs changes from Ross

  tweak SHGetFolderPath
  preprocess: Hugs+MinGW: remove special case
  test *_HOST_OS (as well as *_TARGET_OS, which is there for old GHC's)
  take care of string literals while stripping comments
  rearrange so that we print "Compiling FFI stubs" at most once
  only run ffihugs on Main module if required
  add comment about older Windows versions
  Hugs only: avoid SHGetFolderPath for now
  convert input file paths to platform form (from Brian Smith)
  convert paths to native form at entry
  rename --builddir as --scratchdir to avoid clash with autoconf --build flag.

  doc: consolidate descriptions of preprocessors