Regularize our handling of <sys/signal.h>: currently we include it in
osdep.h, but only for OpenBSD, and we include it without an ifdef
guard in a couple of C files.  This causes problems for Haiku, which
doesn't have that header.

Instead, check in configure whether sys/signal.h exists, and if it
does then always include it from osdep.h.

Signed-off-by: David Carlier <devnexen@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20200703145614.16684-5-peter.maydell@linaro.org
[PMM: Expanded commit message; rename to HAVE_SYS_SIGNAL_H]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>