Move gdbsupport to the top level

[Eli Zaretskii]

> From: Tom Tromey <tom@tromey.com>
> Cc: Tom Tromey <tom@tromey.com>,  palves@redhat.com,  gdb-patches@sourceware.org
> Date: Tue, 17 Sep 2019 10:34:56 -0600
> 
> >> 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.
> 
> Eli> Would it be possible for whoever tars the release to drop the readline
> Eli> into the tree and build it under some opt-in configure-time switch?
> Eli> Making this opt-in might solve at least some of the problems that led
> Eli> to the decision, I think/hope.
> 
> Seems like a good suggestion to me.

Thanks, I hope it will be accepted.

My rationale is that, since GDB uses Readline in a much more
sophisticated way than many other packages, I'd expect GDB to depend
on a relatively recent version of Readline, where several important
changes were made lately, in particular those submitted by us.  Having
such a version ready in the tarball would, on the one side, allow
people not to upgrade their system Readline just to be able to build
and use GDB, and OTOH, will greatly simplify the task of finding that
specific version which GDB requires if they want to build GDB with a
version of Readline different from their installed one.