Upgrade libevent version to 2.1.8

Author: tinycedar

build.sh

@@ -23,7 +23,7 @@ install_lib_dir="${basepath}/bin"
 fname_libevent="libevent*.zip"
 fname_jsoncpp="jsoncpp*.zip"
 fname_boost="boost*.tar.gz"
-fname_libevent_down="release-2.0.22-stable.zip"
+fname_libevent_down="release-2.1.8-stable.zip"
 fname_jsoncpp_down="0.10.6.zip"
 fname_boost_down="1.58.0/boost_1_58_0.tar.gz"