A modification to the way we build link lines.

Currently the link line is constructed like this, for packages p1, p2 etc.:

  libraries p1 ++ extra_libs p1 ++
  libraries p2 ++ extra_libs p2 ++
  ...
  extra_ld_opts p1 ++
  extra_ld_opts p2 ++
  ..

This change makes it follow this pattern:

  libraries p1 ++ extra_libs p1 ++ extra_ld_opts p1 ++
  libraries p2 ++ extra_libs p2 ++ extra_ld_opts p2 ++
  ...

which seems more useful: in particular it means that using foo-config
(eg. gtk-config) to populate extra_ld_opts should now work properly,
and extra_libs is no longer strictly speaking needed (you can just use
-l options in extra_ld_opts and get the same effect).

Also:

  - There's now no difference between -l<lib> and -optl-l<lib>
  - GHCi grabs libs from extra_ld_opts as well as extra_libs