Unused variable fixes related to conditional compilation

Unused variable fixes related to conditional compilation

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.