Add compatibility for old libevent.
Add compatibility for old libevent.

Issue:	#109
Suggested by:	@yellowman

minaevmike/rspamd,andrejzverev/rspamd,AlexeySa/rspamd,AlexeySa/rspamd,minaevmike/rspamd,amohanta/rspamd,AlexeySa/rspamd,minaevmike/rspamd,amohanta/rspamd,awhitesong/rspamd,dark-al/rspamd,AlexeySa/rspamd,AlexeySa/rspamd,andrejzverev/rspamd,AlexeySa/rspamd,andrejzverev/rspamd,dark-al/rspamd,andrejzverev/rspamd,amohanta/rspamd,andrejzverev/rspamd,minaevmike/rspamd,awhitesong/rspamd,andrejzverev/rspamd,dark-al/rspamd,minaevmike/rspamd,amohanta/rspamd,AlexeySa/rspamd,dark-al/rspamd,minaevmike/rspamd,amohanta/rspamd,AlexeySa/rspamd,AlexeySa/rspamd,minaevmike/rspamd,andrejzverev/rspamd,dark-al/rspamd,minaevmike/rspamd,minaevmike/rspamd,awhitesong/rspamd,andrejzverev/rspamd,awhitesong/rspamd