Move gdb/warning.m4 to gdbsupport

This file is used by gdbsupport, gdbserver and gdb, so I think it
belongs in gdbsupport.  Move it there and update the references the
various acinclude.m4 files.

gdbsupport/ChangeLog:

	* warning.m4: Move here, from gdb/warning.m4.
	* acinclude.m4: Update warning.m4 path.
	* Makefile.in: Re-generate.

gdbserver/ChangeLog:

	* acinclude.m4: Update warning.m4 path.

gdb/ChangeLog:

	* acinclude: Update warning.m4 path.
	* warning.m4: Move to gdbsupport.