drop the watcher macro, call start/stop through function pointers and fix a compiler warning.

src/gevent/libev/callbacks.c

@@ -188,7 +188,7 @@ static void gevent_call(struct PyGeventLoopObject* loop, struct PyGeventCallback
 #undef DEFINE_CALLBACK
 #define DEFINE_CALLBACK(WATCHER_LC, WATCHER_TYPE) \
     static void gevent_callback_##WATCHER_LC(struct ev_loop *_loop, void *c_watcher, int revents) {                  \
-        struct PyGeventWatcherObject* watcher = GET_OBJECT(PyGevent##WATCHER_TYPE##Object, c_watcher, _watcher);    \
+        struct PyGeventWatcherObject* watcher = (struct PyGeventWatcherObject*)GET_OBJECT(PyGevent##WATCHER_TYPE##Object, c_watcher, _watcher);    \
         gevent_callback(watcher->loop, watcher->_callback, watcher->args, (PyObject*)watcher, c_watcher, revents); \
     }
 

src/gevent/libev/libev.pxd

@@ -131,6 +131,13 @@ cdef extern from "libev.h" nogil:
         stat prev
         double interval
 
+    union ev_any_watcher:
+        ev_watcher w
+        ev_io io
+        ev_timer timer
+        ev_signal signal
+        ev_idle idle
+
     int ev_version_major()
     int ev_version_minor()