Log Message:
libev: update to 4.31.

4.31 Fri Dec 20 21:58:29 CET 2019
	- handle backends with minimum wait time a bit better by not
          waiting in the presence of already-expired timers
          (behaviour reported by Felipe Gasper).
        - new feature: use timerfd to detect timejumps quickly,
          can be disabled with the new EVFLAG_NOTIMERFD loop flag.
        - document EV_USE_SIGNALFD feature macro.

4.30 (EV only)
	- change non-autoconf test for __kernel_rwf_t by testing
          LINUX_VERSION_CODE, the most direct test I could find.
        - fix a bug in the io_uring backend that polled the wrong
          backend fd, causing it to not work in many cases.

4.29 (EV only)
	- add io uring autoconf and non-autoconf detection.
        - disable io_uring when some header files are too old.

4.28 (EV only)
	- linuxaio backend resulted in random memory corruption
          when loop is forked.
        - linuxaio backend might have tried to cancel an iocb
          multiple times (was unable to trigger this).
        - linuxaio backend now employs a generation counter to
          avoid handling spurious events from cancelled requests.
	- io_cancel can return EINTR, deal with it. also, assume
          io_submit also returns EINTR.
        - fix some other minor bugs in linuxaio backend.
        - ev_tstamp type can now be overriden by defining EV_TSTAMP_T.
        - cleanup: replace expect_true/false and noinline by their
          libecb counterparts.
        - move syscall infrastructure from ev_linuxaio.c to ev.c.
        - prepare io_uring integration.
        - tweak ev_floor.
        - epoll, poll, win32 Sleep and other places that use millisecond
          reslution now all try to round up times.
        - solaris port backend didn't compile.
        - abstract time constants into their macros, for more flexibility.