check libevent compile-time version and runtime version at runtime

Check at runtime that compile-time version (in header file) and runtime version (in the lib memcached linked with) of libevent match. 
If the versions don't match, wired things happen.
In my case, I install both 2.0.19-stable (with Ubuntu apt-get, header files in /usr/include, libs in /usr/lib/x86_64-linux-gnu/) and 2.1.2-alpha-dev in /usr/local. 
If I don't specify --with-libevent=/usr/local when running configure, it seems that memcached use the header file in /usr/local/include and the lib in /usr/lib/x86_64-linux-gnu/. In this case memcached doesn't work, and continuously prints "[warn] event_del: event has no event_base set." when new connections come.