configure: Add a proper check for openpty() in libutil

configure: Add a proper check for openpty() in libutil

On Linux (and maybe some BSDs), we require libutil for the openpty()
function. However, this library is not available on some other systems, so
we currently use a fragile if-statement in the configure script to check
whether we need the library or not. Unfortunately, we also hard-coded a
"-lutil" in the tests/Makefile.include file, so this breaks the build on
Solaris, for example (see buglink below). To fix the issue, add the "-lutil"
to "libs_tools" in the configure script instead, then this gets properly
propagated to the tests, too.
And while we're at it, also replace the fragile if-statement in the confi-
gure script with a proper link-check for the availability of this function.

Buglink: https://bugs.launchpad.net/qemu/+bug/1777252
Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>