haiku: allow passing -pthread as a no-op.

Reduces the need to patch configure/build scripts that assume
`-pthread` is available on POSIX/UNIX-ish systems.

gcc/gcc/config.gcc

@@ -839,6 +839,7 @@ case ${target} in
   default_use_cxa_atexit=yes
   tm_p_file="${tm_p_file} haiku-protos.h"
   extra_objs="${extra_objs} haiku.o"
+  extra_options="${extra_options} haiku.opt"
   ;;
 *-*-linux* | frv-*-*linux* | *-*-kfreebsd*-gnu | *-*-gnu* | *-*-kopensolaris*-gnu | *-*-uclinuxfdpiceabi)
   extra_options="$extra_options gnu-user.opt"

gcc/gcc/config/haiku.opt

@@ -0,0 +1,29 @@
+; Haiku options.
+
+; Copyright (C) 2010-2022 Free Software Foundation, Inc.
+;
+; This file is part of GCC.
+;
+; GCC is free software; you can redistribute it and/or modify it under
+; the terms of the GNU General Public License as published by the Free
+; Software Foundation; either version 3, or (at your option) any later
+; version.
+;
+; GCC is distributed in the hope that it will be useful, but WITHOUT ANY
+; WARRANTY; without even the implied warranty of MERCHANTABILITY or
+; FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+; for more details.
+;
+; You should have received a copy of the GNU General Public License
+; along with GCC; see the file COPYING3.  If not see
+; <http://www.gnu.org/licenses/>.
+
+; See the GCC internals manual (options.texi) for a description of
+; this file's format.
+
+; Please try to keep this file in ASCII collating order.
+
+pthread
+Driver
+
+; This comment is to ensure we retain the blank line above.