Instead of using an OS-specific ifdef test to select the "openpty()
is in pty.h" codepath, make configure check for the existence of
the header and use the new CONFIG_PTY instead.

This is necessary to build on Haiku, which also provides openpty()
via pty.h.

Signed-off-by: David Carlier <devnexen@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20200703145614.16684-3-peter.maydell@linaro.org
[PMM: Expanded commit message; rename to HAVE_PTY_H]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>