libev: use clock_gettime on OS X if available

EV_USE_REALTIME and EV_USE_MONOTONIC, which force libev to use
clock_gettime, are enabled automatically on Linux, but not on OS X. We
used to forcefully enable them for performance reasons, but this broke
compilation on certain OS X versions and so was disabled by commit
d36ba279 ("Fix gh-1777: clock_gettime detected but unavailable in
macos"). Today we need these features enabled not just because of
performance, but also to avoid crashes when time changes on the host -
see issue #2527 and commit a6c87bf9 ("Use ev_monotonic_now/time
instead of ev_now/time for timeouts"). Fortunately, we have this cmake
defined macro HAVE_CLOCKGETTIME_DECL, which is set if clock_gettime is
available. Let's enable EV_USE_REALTIME and EV_USE_MONOTONIC if this
macro is defined.

Closes #3299