Add gdbsupport check-defines script

Add gdbsupport check-defines script

This adds a new script that tries to check that none of the support
code uses defines that are not defined by common.m4.  This check is
necessarily inexact, but this script caught all the issues fixed in
the previous patches.

gdbsupport/ChangeLog
2020-01-14  Tom Tromey  <tom@tromey.com>

	* Makefile.in: Rebuild.
	* Makefile.am (check-defines): New target.
	* check-defines.el: New file.

Change-Id: I59450e91394d5e6a7fa59e9ab53c95843c4bacd9