libev: backport select()'s limit workaround

As stated in the 'OS/X AND DARWIN BUGS' section of the libev
documentation [1], kqueue() and poll() have known problems on Mac OS, so
the library uses select() on Mac OS (it is the build time default). The
library however uses the trick to overcome 1024 fds limit: libev sets
the undocumented macro _DARWIN_UNLIMITED_SELECT, which enables linking
against select() implementation without the limit.

The magic macro stops working at some point around Mac OS 10.10 (see
[2]), because it was defined after <sys/time.h> inclusion.  For recent
Mac OS versions the macro has effect only when it is defined before
<sys/time.h> inclusion.

The macro definition was [moved][3] in libev 4.25. Excerpt from the
changelog [4]:

 | 4.25 Fri Dec 21 07:49:20 CET 2018
 | <...>
 | - move the darwin select workaround higher in ev.c, as newer versions of
 |   darwin managed to break their broken select even more.

More proper fix would be updating of libev to a newer version, however I
would postpone it until a moment when we'll have a time to properly test
everything with a new version of the library.

[1]: http://pod.tst.eu/http://cvs.schmorp.de/libev/ev.pod#OS_X_AND_DARWIN_BUGS
[2]: http://lists.schmorp.de/pipermail/libev/2018q2/002788.html
[3]: http://cvs.schmorp.de/libev/ev.c?r1=1.482&r2=1.483
[4]: http://cvs.schmorp.de/libev/Changes?view=markup



Fixes #3867
Fixes #4673

Investigated-by: Maria Khaydich <maria.khaydich@tarantool.org>
Co-authored-by: Maria Khaydich <maria.khaydich@tarantool.org>