Avoid misleadingly-named g_spawn_check_exit_status() if possible

g_spawn_check_exit_status() was renamed to g_spawn_check_wait_status() in GLib 2.70 because its name was misleading: the first argument was always a wait-status (the output of wait(), waitpid() or waitid(), encoding either an exit status or a terminating signal) rather than an exit status.

Use the modern name in our code. When building for scout or continue to provide our own implementation; when building for anything else older than GLib 2.70, provide a macro to wrap the old, misleading name.