gh-1037: Bundled libev configuration

Enable monotonic timers (if available).

Enable floor() usage in computations (instead of a crude approximation
with bithacks).

Enable eventfd and signalfd usage on Linux.

cmake/BuildLibEV.cmake

@@ -29,6 +29,8 @@ macro(libev_build)
         #
         # Enable Linux-specific event notification API (man inotify)
         set(ev_compile_flags "${ev_compile_flags} -DEV_USE_INOTIFY")
+        set(ev_compile_flags "${ev_compile_flags} -DEV_USE_EVENTFD")
+        set(ev_compile_flags "${ev_compile_flags} -DEV_USE_SIGNALFD")
     elseif (TARGET_OS_FREEBSD OR TARGET_OS_DARWIN)
         #
         # On FreeBSD build libev loop on top of

third_party/tarantool_ev.h

@@ -43,11 +43,13 @@
 #define EV_USE_POLL 1
 #define EV_USE_NANOSLEEP 1
 #define EV_USE_REALTIME 1
+#define EV_USE_MONOTONIC 1
 #define EV_PERIODIC_ENABLE 1
 #define EV_IDLE_ENABLE 1
 #define EV_STAT_ENABLE 1
 #define EV_FORK_ENABLE 1
 #define EV_CONFIG_H 0
+#define EV_USE_FLOOR 1
 #include "third_party/libev/ev.h"
 #else /* !defined(ENABLE_BUNDLED_LIBEV) */
 #include <ev.h>