This patch fixes various unused variable warnings that are related to
conditional compilation.  In these cases, either the variable is now
protected by the same #if as its uses, or the declaration is simply
lowered into the conditionally-compiled block.

gdb/ChangeLog
2018-07-22  Tom Tromey  <tom@tromey.com>

	* windows-nat.c (saved_context): Conditionally define.
	* remote.c (remote_target::remote_btrace_maybe_reopen):
	Conditionally declare "warned".
	* inflow.c (sigquit_ours): Conditionally define.
	(new_tty): Move "tty" declaration inside #if.
	* guile/guile.c (guile_datadir): Conditionally define.
	* charset.c (set_be_le_names): Move some declarations inside #if.
	* btrace.c (parse_xml_btrace): Move "errcode" declaration inside
	#if.
	(parse_xml_btrace_conf): Likewise.