Currently we check PTRACE_PEEKSIGINFO and if it's defined in a system
header, we suppose that ptrace_peeksiginfo_args is defined there too.

But due to a bug in glibc, this check doesn't work. Now we have F20,
where ptrace_peeksiginfo_args is defined in sys/ptrace and F21 where
it isn't defined.

commit 9341dde4d56ca71b61b47c8b87a06e6d5813ed0e
Author: Mike Frysinger <vapier@gentoo.org>
Date:   Sun Jan 5 16:07:13 2014 -0500

    ptrace.h: add __ prefix to ptrace_peeksiginfo_args
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>