Try to workaround bug in libev4.33 on Windows where have_monotonic is not defined.

_setuplibev.py

@@ -75,8 +75,13 @@ def build_extension():
                                        'src/gevent/libev/stathelper.c',
                                        'src/gevent/libev/libev*.h',
                                        'deps/libev/*.[ch]'))
+    # While we don't actually use periodic watchers,
+    # on Windows we need to enable them to work around an issue
+    # in libev 4.33 where ``have_monotonic`` is not defined.
+    EV_PERIODIC_ENABLE = "0"
     if WIN:
         CORE.define_macros.append(('EV_STANDALONE', '1'))
+        EV_PERIODIC_ENABLE = "1"
     # QQQ libev can also use -lm, however it seems to be added implicitly
 
     if LIBEV_EMBED:
@@ -88,7 +93,7 @@ def build_extension():
             # libev watchers that we don't use currently:
             ('EV_CLEANUP_ENABLE', '0'),
             ('EV_EMBED_ENABLE', '0'),
-            ("EV_PERIODIC_ENABLE", '0'),
+            ("EV_PERIODIC_ENABLE", EV_PERIODIC_ENABLE),
             # Time keeping. If possible, use the realtime and/or monotonic
             # clocks. On Linux, this can reduce the number of observable syscalls.
             # On older linux, such as the version in manylinux2010, this requires