Support for libev >= 4.0
multiwatch doesn't compile against libev in some cases where
libev has been compiled with EV_MULTIPLICITY=0 (in debian, with
libev4, because libev3 did not hide ev_loop even in that case).
Configure checks for ev_loop in ev, which is unavailable, and
making it check for ev_default_loop fixes the issue.
So it works in both cases, it is just a matter of how it checks
the availability of libev.
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