Use event_base_new(), not event_init(), to detect libevent 2.

(event_init() is obsoleted in libevent 2.)

Fixes bug 21051; bugfix on 0.2.9.1-alpha when we dropped libevent 1
support.

changes/bug21051

@@ -0,0 +1,3 @@
+  o Minor bugfixes (compilation):
+    - Fix Libevent detection on platforms without Libevent 1 headers
+      installed. Fixes bug 21051; bugfix on 0.2.9.1-alpha.

configure.ac

@@ -508,12 +508,13 @@ TOR_SEARCH_LIBRARY(libevent, $trylibeventdir, [-levent $STATIC_LIBEVENT_FLAGS $T
 #ifdef _WIN32
 #include <winsock2.h>
 #endif
-void *event_init(void);],
+struct event_base;
+struct event_base *event_base_new(void);],
     [
 #ifdef _WIN32
 {WSADATA d; WSAStartup(0x101,&d); }
 #endif
-event_init();
+event_base_new();
 ], [--with-libevent-dir], [/opt/libevent])
 
 dnl Determine the incantation needed to link libevent.