Inching closer to making the HGL part of the standard build.

This commit is mostly to remove bits of xlib and greencard whch had
crept into the source tree.

ToDo:

- make greencard part of standard build (waiting for response from
  simonM)
- make greencard install libHSgreencard when it builds
- find a autoconf-respecting way of installing the package
- only build xlib and HGL if you have X11 installed
- merge all these changes into HEAD
- should the demos/ tests be built as part of the standard build?
  what's the convention here?
  (Note that the tests can't be used in batch more - you gotta click
  on the window.)
- a little debugging - the tests aren't quite working.