[PATCH v2 0/6] Move gdbsupport to top level

[Tom Tromey <tom at tromey dot com>]

Here is an update of the series to move gdbsupport to the top level.
This is one step in the bigger projecct to move gdbserver to top
level.

In this patch, gdbsupport is given its own configure script --
however, gdbserver still builds its own copy.  gdb and gdbserver won't
share a gdbsupport library until the final series.

This version of the patch fixes up the problems that Pedro pointed out
in the shared nat/ and target/ code.  In particular, now they can
simply rely on the shared config.h.  This is enforced by ensuring that
the necessary defines are all available; the checker script I used to
find the issues is provided in patch #5.

I wasn't able to send this through the buildbot.  I did test it on
x86-64 Fedora 29.  I also build it using a mingw cross.

If you want to try it, it is on the branch
submit/move-gdbsupport-to-top in my github.

Let me know what you think.

Tom