Make ax_lib_event.m4 set a per-version variable on success

This makes it a little bit easier to check for several versions of
libevent and see which succeeded.

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@799017 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/aclocal/ax_lib_event.m4 b/aclocal/ax_lib_event.m4
index 08043b4..56aefcc 100644
--- a/aclocal/ax_lib_event.m4
+++ b/aclocal/ax_lib_event.m4
@@ -181,6 +181,7 @@
             else
               AC_MSG_RESULT(yes)
               AC_DEFINE(HAVE_LIBEVENT,,[define if libevent is available])
+              ax_have_libevent_[]m4_translit([$1], [.], [_])="yes"
             fi
 
             ax_have_libevent="$success"