Those aren't errors, they're infomational messages. They're just telling you that the files config.guess, config.sub, and ltmain.sh already exist. Adding the --force switch to libtoolize will cause the existing files to be overwritten. You would have to edit autogen.sh to add the --force switch to the libtoolize command.
Unless you checked out a development version of libevent or made changes that require the configure script to be remade, there is no reason to execute the autogen.sh script. libevent uses GNE autotools and is a simple configure && make && su -c 'make install'.
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