Fixed compilation on NetBSD 10. NetBSD 10 changed struct kevent's udata type from "intptr_t" to "void *", similarly to how it is defined on other systems. This breaks compilation, since intptr_t is assumed on NetBSD. Fix is to limit special intptr_t handling to NetBSD versions before 10.0. To simplify testing, relevant definitions are moved from configure to code (which seems to be the right thing anyway).