Move gdbsupport to the top level

[Tom Tromey]

Tom> That message neglected option 4, which is removing readline from the
Tom> tree.  (But making it continue to work if the sources are dropped in, as
Tom> we already do for libiconv etc.)

We discussed this at Cauldron and agreed to try this approach.  That is,
the plan is to remove the readline sources from the tree, and remove
--with-system-readline, but follow the approach of some other libraries
where one can unpack readline into the source tree and top-level
configure will arrange to build it.

Tom